Immortal Immaturity

""There's no point in being grown up if you can't be childish sometimes.""

- The Doctor, Doctor Who

The tendency of immortals in fantasy and science fiction to not act significantly older than they look -- even if they're Really Seven Hundred Years Old.

Fair enough when you're talking about an alien species or fantasy race with a very long lifespan. Ancient vampires who behave like teenagers -- or even like ordinary adults -- may present more of a strain on credibility, relative to the seriousness of the genre in which they appear.

Often justified in the sense that an immortal wouldn't have to deal with many of the psychological aspects of growing older. Their bodies don't break down with age, so they don't have to come to terms with decreased mobility or mental capacity. They won't die of natural causes, so they don't have to contemplate the inevitability of death, or the possibility of an eternal reward (or punishment) and all the moral responsibility that might carry. They don't have to worry about their legacy or leaving anything unfinished, because there really is always tomorrow. Because of all this, these characters never need to "grow up" in the sense that a mortal does.

A darker take of this would be where the immaturity is a facade, and in fact the character does have inner angsts and turmoils that most mortals actually wouldn't be able to relate to. For example, being immortal but having mortal friends would essentially mean that you would have to watch them age and eventually perish, and while you wouldn't have to wonder about what awaits you in the afterlife, the question of what will happen to you when the world ends and there are no more people remains - thus, inner angst is generated over stuff that mortals don't have to worry about, but is kept hidden except for special episodes when the immortal character gets the spotlight, when such issues are thrown into light for development purposes.

Also, there are those who argue that absent changes to the body, the rate of 'change' to the mind slows even in real life, that is, the changes between birth and age 10 are vastly bigger than the changes from 10 to 20, which in turn are much larger than the difference between 20 and 40 or even 20 and 60. This argument would have it that absent the bodily changes of old age, the changes from 50 to 500 might not be so extensive as one would otherwise expect.

Of course, for immortal children who find Not Growing Up Sucks (or rocks), it may well be that having their clock stopped at a certain age also froze their cognitive development. They may get smarter, but not wiser.

It's also probable that a being with a longer lifespan could take a proportionately longer time to reach emotional maturity. A 100- or even 1000-year old alien might be the equivalent of a human ten-year-old.

Another part of this trope is that very often the apparent ages of immortals correspond to their status, so that the leaders are in their 30s with teenaged followers, even when their apparent age should have no relationship to their true age.

If The Mind Is a Plaything of The Body, then this trope may result as a consequence. See Emotional Maturity Is Physical Maturity for the broader concept of physical age being more important than actual age as regards emotions.

Anime and Manga

 * Chibiusa, a.k.a. Sailor Chibi-Moon, from Sailor Moon looks and behaves like a five-year old. According to the manga, she's about 900. She still seems to find kindergarten challenging.
 * She spent much of this time in suspended animation in the anime, and it's implied that her growth was physically and mentally stunted. Somewhat creepy, but justified.
 * The Wolkenritter from Magical Girl Lyrical Nanoha. Vita looks like a child, and acts like a child -- though it could be fairly said that she's just immature, which is a problem that doesn't automatically go away with age. Her rather sucky role in life up to that point, and during the second season, probably also contributed to her lack of control via lots of emotional pain. Still, it's awfully coincidental that the most child-like Knight in appearance would be the most childish in behavior, given that they're all supposed to be the same age.
 * Despite this, she claims to be an adult when going to a public bath in Magical Girl Lyrical Nanoha Strikers Sound Stage 01.
 * Chizuru of Kanokon. On the one hand, she is a fox spirit, with all the instincts that entails, but on the other, she's four hundred years old, so you'd think she'd know that trying to rape a shota is bad manners. Her mom is a little better but still has entirely too much fun with said shota herself.
 * The Twelve Kingdoms: There are immortal characters who are over a hundred years old, but still act and consider themselves as teenagers.
 * Evangeline of Mahou Sensei Negima likes to act refined (difficult when she has to look up at the person), but sometimes lapses into childish whimpering (usually when she trips and falls). Whenever she starts to subvert this and act her age, Chachamaru usually tells her "please don't start pretending you're old."
 * She even lampshades this on at least one occasion, laughing ruefully about how her pre-pubecent form influences her self-perception and personality.
 * It's also implied that Albireo "Colonel Sanders" Imma is much older than he appears. He spends a good chunk of his post-Tournament Arc appearance making fun of Evangeline. Especially her middle name.
 * from Higurashi no Naku Koro Ni could be considered something of a subversion. Despite being Really Seven Hundred Years Old, her outward behavior and speech patterns are still those of an adorable, cheerful girl. As it turns out though, this is only an act so as not to alarm everyone who's oblivious to Hinamizawa's dark secret. Mentally, she's a rather serious adult. That said, she hasn't grown nearly as old mentally as she thinks she has.
 * Played straight with, who's even older, but is really quite shy and easily flustered.
 * She can be quite serious and mature though. The manga tends to make her overly mature, and the anime tends to make her too immature.
 * Hinako Ninomiya of Ranma 1/2 is somewhere in her mid-to-late twenties; however, thanks to Happosai, her body ages incredibly slowly, so she currently looks like a ten year-old girl. She can only grow up to reflect her true age by absorbing the Battle Aura of highly-combative people around her, and even then, only temporarily. How she acts depends entirely on how she looks -- she could be watching Doraemon, reading Shojo or gushing over the giant panda in the living room as a child one second, only to suck out Ranma's ki and try to hook up with Akane's father as a grown, mature woman the next.
 * Don't forget Happosai, whose general outlook on life can be summarized roughly as "five year old child with the hormones of a stereotypical boy just hitting puberty".
 * In Mermaid's Scar, a subversion occurs when an immortal who has lived for 800 years as an 8 year-old boy only acts as a child to keep up appearances; his mind is anything but childish.
 * Some of the Soul Reapers from Bleach, particularly Rukia and Renji, alternate between acting like the teenagers they appear to be and the Really Seven Hundred Years Old warriors they actually are.
 * Lety from The Record of a Fallen Vampire is Really Fifty-Six Years Old but looks and acts like child (well, a child with incredible sniper skills). Her excuse: if she acted her real age it would appear "unnatural" and she'd have a harder time hiding among humans.
 * Pretty much every nation-tan in Axis Powers Hetalia. Most of the characters have a "human age" of around early teens to late twenties, but being anthromorphizations of nations they are all hundreds or thousands of years old. They usually act more like their human ages (and sometimes even younger). Taken to the extreme with the bubbly and youthful China, listed with a Human age of 4000 and immortal.
 * Wait, Inuyasha is not about fifteen?
 * In Fullmetal Alchemist, Greed has a habit of referring to older humans in a disrespectful way ("old fart", "pops", and the like), which fits this persona/appearance as a punk in his early twenties. However, it's kind of odd when you consider that he's closer to three hundred years old.
 * And
 * Kyou Kara Maou's Wolfram von Bielfield is eighty-four, and not only does he behave like a particularly bratty and childish fifteen, sword skills he's been honing for sixty years can't stand up to a determined baseball swing from Ordinary High School Student Yuuri, if Wolfram loses concentration in rage.
 * Can be justified along the rationale that Mazoku learn very, very slowly, and this is the only reason (along with Houryoku allergy and low birthrate) they haven't conquered the world or made incredible strides in the arts and sciences that put everyone else's cultures to shame.
 * Cheri, Anassina, Stoffel, and to a certain extent Gunter and even Hube show symptoms of this. Gwendal on the other hand seems to have been an old man by twelve sixty.
 * The main devils of The World God Only Knows. Elsie's about 300 and behaves like a ditzy 12 year old. Her classmate Haqua is a little better - but only a little, being about as emotionally mature as a human teen. It's a reliable source of humor in the scenes focusing on them.
 * To be fair, the implication is that they basically are human teens. They're at the stage of just having graduated and taking up bottom rung jobs.
 * Mina Tepes from Dance in The Vampire Bund seems to suffer from this intremittently. It is never made clear how much is an intentional put on, how much is her relaxing when not obliged to put up a strong front, and how much is neither; but there are times it is hard to remember that she is older than fifteen... and other times when it is hard to mistake her (despite her height and figure) for anything younger than fifty.
 * Holo from Spice and Wolf looks and usually acts like a girl in her teens despite being a harvest god hundreds of years old.

Comic Books

 * All of the Endless in Neil Gaiman's The Sandman display this to a greater or lesser extent: Delirium is eternally childlike, and even dignified Dream has his moments of angsting like a lovelorn teenager. On the other hand, Death looks and talks like a teenage Perky Goth, but she is also one of the most mature and philosophical characters in the series and clearly knows the weight of her age and responsibility.
 * In fact, later issues imply that Death matured to become the quasi-teenage Perky Goth we all know and love: in her "youth," billions and billions of years ago, she was cold and cruel.
 * Similarly, Delirium is quite capable of acting like an adult when she needs to. It just really, really hurts her to become something she is not. Whether it is a physical or mental pain is never stated, however.
 * Delirium's introduction cites her tragedy as being "forever the youngest of the Endless", despite the fact that she is older than all of humanity. This is only one interpretation, however.
 * One of the subplots in the latter part of Preacher is the general immaturity of the vampire Cassidy, and its destructive impact on not just himself but the people he comes into contact with.
 * Somewhat back and forth in Fables. Most of the characters act like the thirty-somethings they are physically, except for Pinocchio, who, much to his annoyance, is still pre-pubescent. (Though for some reason the artist is fond of drawing him with a rather strong jaw.)
 * In Tom Strong, Tesla Strong, despite really being seventy-five years old, looks, acts, and is treated like a teenager.
 * , the vampire foe of the Runaways, is an aversion. He hasn't aged past being a teenager in 100 years. Hasn't stopped him from making a killing on the stock markets in the 1920s and losing it all in the Dot Com bubble.
 * In The Super Hero Squad Show comic, Eternity is revealed to be an avid collector of entities.

Fan Fiction

 * Nobody Dies plays with this trope. Even though Sandalphon is only about 16 years old, he IS the Angel of Tears (complete with Creepy Monotone and Voice of the Legion). And he fist-bumps with a human. Also, Iruel's reaction upon seeing Rei's personal hallucination involving Unit 05: "What. The. F___." Hell, Rei herself.

Film

 * In Terry Jones's film Erik the Viking the Norse gods are petulant children who don't care about mortals.
 * Mr. Magorium of Mr. Magoriums Wonder Emporium certainly counts. Of course, he does owns a magical toy store...
 * In Dogma, God is mentioned as being "a Skee-Ball fanatic". When we finally see her, she spends some time doing a handstand (and falling).
 * Then again, there's a difference between being mature and having no sense of fun.
 * Let the Right One In features Eli, a vampire who has been twelve for 'a very long time'...but still acts pretty much exactly like a twelve year old. It is widely debated among fans, however, just how much her immaturity is the real Eli and how much is an act put on for the sake of getting a new Renfield.

Literature
""You call yourself some kind of goddess and you know nothing, madam, nothing. What don't die can't live. What don't live can't change. What don't change can't learn. The smallest creature that dies in the grass knows more than you. You're right. I'm older. You've lived longer than me but I'm older'n you. And better than you. And, madam, that ain't hard.""
 * In The Hitchhikers Guide to The Galaxy, Wowbagger the Infinitely Prolonged has made it his mission to insult every being in the universe. Personally. In alphabetical order.
 * Which only began after the awesomeness of being immortal finally wore off and the eternal boredom set in.
 * Lampshaded in Ken MacLeod's novel Newton's Wake: A rejuvenated woman says people like her just get a bit "cannier", and passes the rest of it off as fatigue poisons and neural decay.
 * Peter Pan is all about this, his skills with a blade on par with those of Captain Hook notwithstanding. Justified in his case, since his overwhelming desire to be immature is why he became immortal in the first place.
 * In Dragonlance, Paladine, the god of good, when in his avatar form is rather a comic figure. Some of that might be Obfuscating Stupidity but it is implied that some of it is also his real character.
 * Paladine's more Eccentric Mentor than anything. He may act eccentric and amusing, but even as Fizban he's always on top of what's going on.
 * In Harry Turtledove's Fox series, the gods are SpoiledBrats because, being nearly all-powerful, no one can discipline them. The only exception is the All-Seeing Ikos, and even he can be manipulated by his pride.
 * The gods of David Eddings' Elenium and to a significantly lesser extent Dweia from The Redemption of Althalus are prone to some rather immature emotionalism. It's explicitly explained by the world-weary Sparhawk and the worldly con man Althalus in both cases as almost stemming from their immortal nature; having never needing to fear death or be chastised they don't develop maturity as well in some aspects of their personality.
 * Curiously, the least immature Styric deity in Sparhawk's world seems to be Aphrael, the Child Goddess. One of her brothers, Setras, appears more concerned with sunrise artwork than with humans and has to have every plan explained to him in words of one syllable bit that words are made out of.
 * Edward from Twilight. Basically he acts like a teenager.
 * A more frightening take on this is the immortal children: a child who becomes a vampire will never be able to gain the self-control of an adult vampire, and will instinctively slaughter everything instead. Combine that with super-vampire strength, speed, etc. and the supernatural ability to make anyone who lays eyes on them fall obsessively in love and fight to the death to protect them, and it's no wonder it's illegal to bite children.
 * Lampshaded with the "teenaged" vampires in Discworld's Carpe Jugulum, who are three hundred years old, and wish their parents would treat them as such... but don't act it.
 * Also, Granny Weatherwax to the Queen of Elves in Lords and Ladies:


 * Discworld's gods tend to be rather lacking in maturity as well as brains. (Thinking is what worshipers are for.) Small gods, who haven't attracted any followers, are even moreso: a pinch of ego and some willpower.
 * Sam and Hailey from Only Revolutions, who are "allways [sic] sixteen".
 * Avoided at first with Eli, a vampire who's been twelve for a long time in Let the Right One In. She doesn't act or think like a child, but begins to (possibly subconsciously) do so when she starts hanging around with the main character.
 * In the Ethshar novels, eternal youth spells freeze someone at that point in their maturity, and immortal wizards who revert themselves back to youth note that a younger body makes them act differently (especially as regards the libido.) One immortal who's been 15 for 200 years is eventually dumped by her 16-year-old beau as being too young (emotionally speaking) for him.
 * It's specifically noted that her emotional immaturity results from her miscasting the eternal youth spell: essentially, she's cursed to never grow up. Other eternally young wizards behave more like an old person in a young body.
 * Nearly every freaking god in Everworld is basically a five year old in terms of maturity and level of trust. This is a combination of naivete, an inability to change, and a lack of survival instinct. Some of the few exceptions include Loki, Huitzilopochtli, Athena, and Ka Anor, the God Eater.
 * The fey in Wicked Lovely. They are immortal, or at least Really Seven Hundred Years Old, but they very rarely act or look anything more than early-twenties. Probably the best example is Keenan, who is a 900 hundred year old teenager in pretty much all respects.
 * Honor Harrington occasionally addresses the implications of slowing the aging process and how much of your personality is influenced by hormone levels based on your age. Puberty and its associated mood swings has years added on to it.
 * In Chronicles of Amber, most of the Amberite Princes initially think and act like charmless Royal Brats, but as the series goes on and conflicts force them to work together, develop into The Wise Prince.

Live Action TV
"Kathy: I'm 3000 years old! When are you going to stop treating me like I'm 900?"
 * As mentioned in the page quote, the Doctor, from Doctor Who. The Eleventh version's claim to be "universally recognised as a mature and responsible adult" is played as the biggest lie he's ever told, given that it shorts out his psychic paper (the same paper that once convinced a stranger he was the King of Belgium). This isn't exactly a new development, either: the Fourth Doctor ran around munching on candy and the First started out acting like a spoiled teenager.
 * In the revival series, the Master is something of a Psychopathic Manchild, to help cement him as the Doctor's Evil Counterpart.
 * Less straightforward example: Older vampires from within the Buffy the Vampire Slayer universe tend to project "moderately clever adult who's been around the block a few times" more than "old man". Angel's interest in looking cool is occasionally lampshaded as age-inappropriate, despite the series stressing his maturity relative to the teens and 20-somethings he hangs out with.
 * Anya was also something of an interesting case. Her behavior was explainable at first due to her life as a demon that had no relationship with humans. When it was eventually revealed that she was a human at one point in time, however, the explanation was retconned simply by demonstrating that she was always just plain weird. Spending eleven hundred years as a demon probably did not help.
 * Spike, a vampire over a hundred and forty years old, mopes like a teenager when Drusilla breaks up with him. He gets some motherly advice from Joyce Summers, a human woman a fraction of his age. (The fraction in this case being 'less than a third but more than a quarter'.)
 * Buffy's original college roomate, Kathy. She acts like a typical college freshman, but it turns out she's a demon and thousands of years old. (She's also in big trouble from her guardian.)


 * The teenage vampire on Moonlight in the episode "Arrested Development". He was 200 years old and acted like an angsty teenager. It was blamed on permanent hormones, but the writers seem to have forgotten that 200 years ago, someone his age could get married and would be treated as, and act as, an adult member of society.
 * In the TV shows Hercules: The Legendary Journeys and Xena: Warrior Princess, the gods are pretty much just 30-year-olds (or even 12-year-olds that look 30) with big egos.
 * Which is either a case of Did The Research or a coincidence, as the Greek gods were often portrayed as petty and cruel in the original mythology, as noted below.
 * Trelane, in the Star Trek the Original Series "The Squire Of Gothos," was an extremely powerful near-god and hundreds if not thousands of years old, but acted like a child...and by the standards of his race, he was--his parents showed up at the end to drag their whining kid home.
 * Q from the later series was similar, but more in line of the Hercules and Xena gods: looks to be 30, huge ego, power to match.
 * And, of course, Peter David would make Trelane a Q (non-canonically) in the novel Q-Squared.
 * Jadzia Dax, though not technically immortal, has several lifetimes of experience. Yet you wouldn't know it if they hadn't told you about it.
 * Averted with Highlander: The Series. Given the way they live -- constantly on the lookout for people with swords, forced into kill-or-be-killed duels, watching mortal friends grow old and die, moving around to avoid detection -- most Immortals trade their immaturity for a more practical outlook on life. The only immortal child shown uses his apparent innocence as a honey trap so he can kill other Immortals.

Mythology

 * The gods in many religions seem to be rather immature.
 * Sometimes this is a case of giving educational tales, using gods as examples, rather than making the gods outright immature - Hinduism is filled with stories like this for example, despite that in their canon gods are too great to even directly interact with the mortal world.
 * The ancient Greek gods are constantly interested in the affairs of mortals; they also breed with mortals. They don't seem to learn anything from experience, as far as wisdom is concerned. They are petty, cruel and constantly fighting amongst each other.
 * The train of events that led to the Trojan war began with Eris getting pissed over not being invited to a wedding. The modern day religion of Discordianism uses this tale as its jumping-off point.
 * It's worth noting that this is a shifting trope - in the Classic period the Greeks considered their gods to be akin to humans, but morally perfect. Then they got hit by a severe case of Flanderization, and by the time Homer got his hands on them, they were a bunch of immature morons; perhaps a case cultural cynicism towards the arbitrariness of nature's forces had hit them hard.
 * Since Homer lived several hundred years before the Classical period, the trope shift was in the opposite direction.
 * D'oh!
 * There is a difference, however, between the gods in the myths and the way they were thought of in religious ceremonies. The Greeks also saw their gods as having multiple aspects that were so different that it was possible to be perfectly devoted to and blessed by one aspect of Zeus while being on another aspect of Zeus' bad side. Compare the difference Between God as He/She/It is worshipped by the Abrahamic faiths with God's character in The Bible and Koran.
 * God doesn't appear much as a character in the Koran...
 * A better comparison would be God in the Old and the New Testament, as well as contrasting between Abraham's and Moses's relationship with Him - the difference is so massive that some scholars believe that Abraham's deity was entirely different from Moses's, and they were interpreted as one and the same through an Orwellian Retcon.

Tabletop Games

 * Vampires in White Wolf's Vampire: The Requiem tend to be "frozen" at where they were when they first became Vampires. They do become more knowledgeable and powerful, but they keep the same prejudices and emotional maturity they had from when they were still alive. Therefore, even though a vampire may be 200 years old, he could still act like a hot-tempered 17 year old.
 * Given that most Vampire players are 17 years old, this is entirely appropriate.
 * Somewhat justified in WoD terms in that vampires are inherently creatures of stasis, who are effectively frozen at the moment of their Embrace. While they are capable of maturing and evolving over time, the process is far, far harder than it would be for a living being. Thus, vampires tend to calcify into the personalities (and emotions, and prejudices, and general world-view) they had when they were Embraced. Part of why it's considered a horrible thing to turn children into vampires - they will never mature, either physically or mentally.
 * The status part of the trope is also inverted. Danse Macabre points out that since hundreds of years ago, kids became working parents in their mid-teens, most elder vampires would be teenagers. Younger vampires with lower status would become progressively more physically older, corresponding to human societal advancement. This explains why vampire politics so closely resemble high school cliques: for all intents and purposes, the elders are perpetual high school students. Who can tear your head from your shoulders with a flick of the wrist.
 * Subverted in the Ravenloft supplement Van Richten's Guide To Vampires, which proposes that vampires go through a psychological maturation-process which commences when they become undead, and is entirely independent of their mortal age. An octogenarian who becomes a vampire can regress to teenage-brat behavior, while a child-vampire who's (un)lived long enough can become a sober, calculating Chessmaster.
 * The 3.5 edition D&D supplement Libris Mortis has this as one of the cons of becoming a lich. While a lich has an eternity to learn, they can't change. They'll always have the same mindset and beliefs they did when they transformed.
 * For changelings in Changeling: The Dreaming this trope is essential if they want to keep their Fae side intact. Though changelings tend to age much more slowly than other humans, eventually Banality will weigh heavily on their Fae souls and they become Undone. They constantly struggle to resist the pressures of growing up to hold on to their magic and immortality for as long as they can.
 * Changelings are basically humans who have been given an Immortal Fae soul. Said soul may have been a powerful being in of itself and is merely body-hopping to survive. The humans they inhabit tend to be quite young. So it is possible, and quite common, to have a millennium old legendary dragon-slayer, a highly respected duke of the Seelie Court, or a highly valued scholar of ancient lore sitting in a living room playing with toy trains until mommy calls him for bedtime.

Video Games
"Tenshi: Every day is singing, singing, drinking, dancing, and singing again, over and over... I was bored out of my mind! So I came down to the surface and watched you playing with all kinds of different Youkai... Seeing that made me think, "I want to play disaster resolution too." So I caused one. A disaster, I mean.
 * Xenogears features several characters who are quasi-immortal. Some have lived for hundreds of years, a few for as many as ten thousand.  act as one might imagine. The former is a world-weary, very old man who has kept his moral compass intact due to guilt over a sin committed long ago, the former being very anxious to transcend to a higher level of existence and possessing of no moral compass whatsoever. Then, there is , who has lived as long, but in a different manner: she possesses a new body when her current one dies. When we meet her, she has the body of a beautiful young woman and appears to act like one, being flirtatious and sexually-active. Yet, it's difficult to tell how genuine this is, because her entire existence is devoted to executing the mother of all plans, and everything she does serves this end.
 * In the Touhou series, the vampire Flandre Scarlet has a justified reason for typically being portrayed with a naive childish mentality: her sister Remilia has kept her locked-up in the house for the last 500+ years. She never gained perspective about the world. (And a bit insane on top of that.)
 * Remilia herself is said to have the personality of a selfish, spoiled brat. She can get serious if the situation really merits her attention, and is actually remarkably on top of things, but her behaviour is that of a spoiled nine-year-old about 95% of the time.
 * Kaguya Houraisan and Fujiwara no Mokou have kept up a thousand year war and kill each other daily purely out of spite. That's a bit childish.
 * Also, they're both immortals with nothing better to do with their time, so it's less spite than a game between them.
 * Then there's Yukari, who despite being more than two millenia old can have her entire personality summarised with the words "for the lulz".
 * All fairies are excellent examples of this trope.
 * Fans think that this may be Tenshi Hinanawi's problem. Most Celestials achieve their status through diligence and careful introspection in their lives, as well as the ability to fight back against death; her clan was quickly whisked up to the heavens by a kami for good work maintaining geologic keystones.

Tenshi: I don't want to go on living the boring life of a celestial forever."


 * Mother 3 has
 * Fate/Stay Night: Saber is, we are told, not actually fifteen; after receiving her sword, she didn't age over the next ten years, and she's really in her twenties. Not only does nobody mention how strange it is for her to have a romance with the seventeen year old main character, but she's as naive emotionally as a real seventeen year old, which is Hand Waved by claiming that she had no personal life.
 * Ciel from Tsukihime has a similar situation going on, but in that case it's actually lampshaded in Kagetsu Tohya.
 * Arcueid Brunestud, of the visual novel Tsukihime, is also much like the previous entries. She's an ancient True Ancestor, but she was bred to be a weapon and not a person by her own race, and so they kept her asleep all the time except when she was sent to kill someone... so her demeanour on the outside world is not unlike that of a curious and confident child, marveling at everything new she comes across and being generally naive and emotionally outspoken. She really isn't any older emotionally than she acts.
 * The angels and demons in the Disgaea series are all over a thousand years old, but since time flows differently in their dimensions, they age 100 times slower then normal and act exactly as old as they look.
 * Xiaomu of Endless Frontier and Namco X Capcom. She may be a 700+ year old sage fox, but she can be immature about certain things, enough to be even spanked for them.
 * Prishe from Final Fantasy XI is a Hot Blooded Tsundere. Granted, she's only in her 30s chronologically, but she doesn't act 30.
 * All of the four goddesses of Hyperdimension Neptunia exhibit this trope, one way or another.

Web Comics
"Misa: "Being the youngest of a race that lives forever means being eternally treated as the baby.""
 * The undead warlock Richard from Looking for Group borders on Man Child at times.
 * El Goonish Shive takes the effort to make it a Justified Trope as the immortals erase their own memories to avoid the complete boredom involved in knowing everything.
 * Played straight with Chaos, the all powerful nobody-knows-what with the attention span of a gnat who manifests as a Creepy Child.
 * Its pretty clear that she's an immortal who refused to "die" (the erasing memories thing); the immortal who explained the whole thing mentioned that they get "more powerful and less sane" as time passes, which describes "Pandora Chaos Raven" (self-named) pretty well.
 * The angels Rumisiel, Cassiel, and Vashiel from Misfile range from 437 years old to possibly several thousand years old and yet still act like teenagers.
 * Especially Rumisiel and Cassiel; Vashiel is somewhat more believable, just deeply honest (to the level that it's physically impossible for him to lie) and highly clueless about stuff he hasn't paid much attention to in his long history of smiting.
 * Also horny and uncomfortable with it, but since angels in this setting are both immortal and an actual breeding species and he's clearly in his breeding years, this makes some kind of sense.
 * Also also making a certain level of stop in regressionville, given that he has ancient guilt issues over destroying a city (and hasn't visibly aged since then, unlike his former comrades) and officially is on vacation. During what is now a celestial emergency. With his useless brother. Sleeping on a couch in brother's girlfriend's basement. The man is practically having a midlife crisis.
 * Both used and subverted in Sluggy Freelance. The magician Kesandru has grown more mature over the centuries. However, he also still seems to think that pulling a rabbit out of his hat should be enough to frighten people into submission.
 * Misa from Errant Story is about 1,500 years old yet frequently acts like she's 15. She has been doted on her entire life though.


 * In Order of the Stick Xykon is not particularly mature for an immortal lich with more than a century of life behind him. Start of Darkness reveals that he was never very mature to begin with, even as an eighty-year old man. He even lampshades how, even at his advanced age, his "life's wisdom" boils down to little more than "sure, being a badass villain is a barrel of laughs, but what's the point if you can't even enjoy the little things in life" (which to him is mostly watching people die gruesomely and drinking coffee). That said, Xykon can be surprisingly clever and patient if he wants to.
 * Redcloak is another example, played in a different way. His aging processes has been retarded by the magical cloak he wears, and he's eventually called out on this by his (normally aging) younger brother: He's spent most of his time following the Dark One's directives (and later, Xykon's) and hasn't used any of that time to actually live a life; and because he never ages, he hasn't been forced to move on from the tragedy of their past the way Right-Eye had to.
 * Mab and most of the other Fae from Dan and Mabs Furry Adventures, sort of. Essentially everything they do is a matter of "It Amused Me," but they operate and think in an entirely different way from anyone else, and evidence suggests that they can also be mature if that's what amuses them at the time. Additionally, since they're immortal and have a flexible view of time, this behavior might be a way to avoid going totally insane. There is also a somewhat sinister slant to their apparent immaturity, especially given their general view of things as being a game of sorts, and that fact that despite near-omnipotence they will still allow their friends to die in preventable ways.
 * This behavior also appears in some of the mortal-but-extremely-long-lived characters. Abel and Kria don't always act like they are in their 400's. Dan and Regina have some excuse though since they actually are in their 20's.
 * Incubi and succubi in general are creatures of emotion, and apparently have a harder time controlling their own in turn, though the clan leaders are more than capable of being serious - perhaps because they are millenia old. Demons have no such excuses, however.

Western Animation

 * Futurama parodied this trope in the episode "Where No Fan Has Gone Before": it turns out that Melllvar the "immortal" energy being is only 34.
 * Played for laughs in the opposite direction though - when Melllvar's mother drags him off to dinner and Fry makes a Star-Trekky comment about how they thought he was a mighty, god-like being, but he was only a child (see Trelane, above). Then his mother says, "Child? He's thirty-four," playing up the Basement Dweller trope.
 * Disney's Gargoyles
 * Demona. Macbeth doesn't suffer from the same problem, having accepted life and its troubles more than she does. (Macbeth is a somewhat dodgy aversion. The event that granted them their immortality turned him into an old, if fit, man at the same time.) Demona, by the way, was an old woman at the time, and part of the deal had them swapping ages. And even when she was an old woman, she was still an immature punk, while Macbeth was fairly mature as a young man.
 * The Children of Oberon frequently do act like children: immature, god-like, arrogant children. As Greg Weisman said, they have "Great power. Little or no responsibility," in addition to being immortal, and apparently never needing to worry about disease or even feed themselves. They get to spend their whole lives playing games and fighting each other. They aren't necessarily evil, but even the best of them don't actually grasp concepts like mortality.
 * In Fosters Home for Imaginary Friends, the imaginary friends don't age, either mentally or physically. Since they were made to be childhood companions, they're almost all intrinsically child-like.
 * This also applies to Madame Foster, who, in spite of being this trope under her old lady demeanor, strangely enough imagined one of the only serious and straightforward friends in the series.
 * Fridge – All of the other imaginary friends leave their creators and move on to be friends to other children. It's entirely possible that because Madame Foster never left her Funny Bunny that he aged with her. Possibly the same thing may happen with Bloo.
 * Takua, one of the oldest beings in the Bionicle universe, likes to put light crystals on his head and run around pretending to be a bunny rabbit.
 * On Adventure Time, thousand-year-old vampire Marceline offhandedly comments (in song) that she's not a mean person, but has "lost track of her moral code" at some point over her long life. She seems to find it over the course of other episodes, though she still runs largely on It Amused Me and Just Joking Justification.