Emotional Maturity Is Physical Maturity

In fictional works regarding either races that are unusually long or short lived, one pattern continually crops up- regardless of a character's real age, they will always have an emotional maturity matching their physical maturity. A one hundred year old elf who looks like they're in their thirties will always act like they're in their thirties. A child who has grown to physical maturity in a few weeks will be exactly as intelligent and oftentimes more so as their physical appearance would imply.

Where this trope can get really funky is when an Interspecies Romance is involved, particularly the Mayfly-December Romance variety. While people in Real Life generally cringe at the idea of a man in his 70s romancing women in their early 20s, this becomes immediately palatable in fiction as long as they physically appear to be the same age. Whether this says more about the motivation of Squick in Real Life or fantasy writing in general is hard to say.

Naturally, this can be explained, to an extent, with Bizarre Alien Biology and differing cultural customs. Not having known of any other intelligent life forms, we can't rightly say for certain how or if emotional maturity would progress with creatures that have a different lifetime than we do. In many cases, though, it still requires that younger members be an Instant Expert in order to learn as quickly as they do.

Very common in stories featuring intelligent robots, where robots only a few years of age will have grasped everything important about human society and interactions. They tend to get a pass, of course, for not really looking a specific age (well, most of the time).

See Immortal Immaturity for the sub-trope describing this phenomenon specifically as it relates to the attitude and emotional quotient of exceptionally long-lived characters. This trope is more focused on the comparative interpersonal interactions of multi-generational casts.

Contrast Not Growing Up Sucks, which inverts this by showing that a 50-year-old in a 10-year-old's body will, in fact, act like a 50-year-old. Compare Age Is Relative. Compare Acting Your Intellectual Age, where intellectual maturity automatically equals emotional maturity.

Anime & Manga

 * Sasha in Uchuu Senkan Yamato is adult at around 1 year old, with full language skills.
 * Mahou Sensei Negima zig zags this one with Evangeline, who constantly flip-flops between a "seen-it-all" mentality (she's Really Seven Hundred Years Old) and the emotional maturity of a child (because The Mind Is a Plaything of the Body).
 * Chachamaru also counts, being roughly 30 months old at the start of the story, but still being one of the most (relatively) normal students in the class. Sayo comes at this trope from the opposite direction, as she's technically been around for over 60 years, but still behaves like a teenager.
 * Averted in Elfen Lied, where Silpelit diclonious characters grow to physical and intellectual maturity in half the time of a normal human, but develop emotionally at a normal rate. For example Nana, who is chronologically 6 years old, has the physical maturity of a 12 year old but behaves as if she is six.
 * Haku in Spirited Away is a centuries old river spirit/dragon. Hints of romance between him and the 10 year old protagonist are made to seem okay because he looks like he's the same age.
 * Partly justified in that, this being Ghibli, their relationship is about as innocent as it gets and could easily be interpreted as platonic.
 * Wolfram from Kyo Kara Maoh is an 82-year-old demon who looks maybe 16 and acts like an even younger bratty teenager.

Comic Books

 * Happened with Gwen Stacy's children in the Spider-Man story Sins Past, who grew up fast by Applied Phlebotinum because Comic Book Time means they would only have been 9 years or so old. Imagine this cover with a 9 year old.
 * There was a clumsy attempt to invoke this trope when Hal Jordan hooked up with the teenage Arisia; Arisia unconsciously used her ring to make her body become mature, and claimed that her race matured fast emotionally, which would make the relationship okay by this trope. Readers who still remembered that Arisia recently looked and acted teenage couldn't accept this, which led to Arisia's actual age being retconned an number of times. It ended with her age being 240 in Earth years, so it is now her teenage form and not her adult form that fits the trope. The current status of Arisia's age in the New 52 is unknown.

Fan Work

 * In the first part of the Harry Potter fanfic Backward With Purpose, subtitled "Always and Always", this is discussed by Harry and Ginny. They've both gone back in time to fix things, and they're trying to figure out what is an appropriate age to resume having sex. They ultimately decide that most of the dangers of teen sex (with proper protection, etc, naturally) are emotional rather than physical, and settle on fifteen years old. Agree or disagree, at least they addressed the issue.

Literature

 * This trope is referenced early in Down and Out in the Magic Kingdom: The hero's girlfriend is 23 years old, roughly a quarter of the time he's been alive. For the duration of the book, he has a body of about the same rough physical age. This is lampshaded, as he acknowledges that earlier in his life, this fact probably would have bothered him.
 * Twilight is a painful case. Ignoring Edward and Bella (which if you accept physical and mental age stay the same, can make a lick of sense, they mention that two year old vampires are forever two, still mildly cringe worthy) But then you have the last book couple. A 22 year old man and a 7 year old girl, and it's okay with her parents.
 * Played with in Thief of Time, in which the abbot of the History Monks was recently reincarnated as a baby boy. While he can usually speak with a wise old man's intellect, his dialogue keeps being interrupted by his toddler-body's overriding, infantile demands for Bikkit!

Live Action TV

 * Appears in Star Trek: Voyager with the Ocampa, who are unusually short-lived but possess cognitive faculties greatly in excess of what you would expect from a race where nearly all the members are less than 10 years old. This gets exceptionally extreme when Harry Kim marries Kes' daughter, who's only 2 years old.
 * It also shows up with the Doctor, a computer program with no sense of personal attitude or interaction at all who, for most of the show's run, is emotionally indistinguishable from a normal man in his mid-40s.
 * The exceptions are the early episodes, where he hasn't really mastered the basics of human interaction yet and consequently comes off as rude.
 * Shows up in Star Trek: Deep Space Nine with the Jem'Hadar, who rapidly age into physical maturity and, in the same time frame, master the higher levels of intelligent thought.
 * Not to mention Jadzia and Ezri Dax, who hardly ever demonstrate their centuries worth of wisdom and experience
 * Averted for laughs in 3rd Rock from the Sun, where the aliens' body assignments were done independently of their real age. As a result, even though Tommy is the teenager, his alien mind is easily the oldest of the group. Consequently, Tommy is the most emotionally capable member of the crew, and it often falls to him to give the others advice when they've dug themselves into a deep hole. This leads to some hilarious visuals, like the time Dick (officially Tommy's father) was sobbing like a child and Tommy had Dick sit in his lap while he consoled him.
 * Of course, when Tommy himself gets in trouble misunderstanding human culture, he often gets the worst of it since he has no one available for help.
 * Liam Kincaid in Earth: Final Conflict is born in the second season and grows to adulthood within one episode, at which point he's written as and treated as fully adult.
 * In Wizards of Waverly Place, Juliet is a 2000 year old vampire who is the girlfriend of the 17 year old Justin. However, she looks and acts like a teenager, and even interacts with her parents like one, making it seem okay.
 * In the first episodes of Buffy the Vampire Slayer season 4 Buffy's demonic roommate looked (supposedly) and acted like a teenager. When her guardian appeared to take her back to hell she complained that she was 3000 years old, but he still treated her like she was 900. Averted with Colin, who was actually more reserved than The Master.
 * Used to odd effect in Doctor Who, whenever there is more than one Doctor present in a story, the one who is physically oldest will generally take charge. In The Three Doctors and The Five Doctors, this means that the one who acts with greatest authority is the First Doctor, and in The Two Doctors the Second Doctor takes charge over the Sixth. Though it could also be argued that the Doctor was maybe genuinely more emotionally mature and experienced in his first form, and has actively behaved more childish with every progressive regeneration...

Video Games

 * Harukanaru Toki no Naka de manga has an interesting subversion. Abe no Yasuaki is an Artificial Human created about two years prior to the start of the story, who looks like an adult (his "official" age is 21), for the most part acts like one, and possesses sufficient amount of knowledge. The catch? He is initially devoid of emotions, and his understanding of anything related to these tends to be on the level of his real age. Akane actually compares him to a small child on several occasions before learning of his origins.
 * Both played straight and subverted with Human!Hakuryuu in Harukanaru Toki no Naka de 3, an ancient deity in the form of a ten-year-old boy whose behaviour is more or less appropriate to his physical appearance; it is implied that he simply has no experience with living among humans—hence the childlike innocence which he retains even after achieving a more adult form.
 * Adult tarutaru from Final Fantasy XI look very young to the human eye and are very commonly characterized as immature. Particularly striking with the Chebukki siblings in Chains of Promathia; the cast includes every race and nationality seen up to that point, and the tarutaru representatives are nominal young adults... who are a bunch of bumbling, mischievous brats whose subplot is obsession with finding their parents.
 * In Disgaea, do 1313 year old demons act and look like they are 13? Yes, yes they do.
 * Tends to be the case for the characters who are dragons in human form or part dragon in the Fire Emblem series, despite being as much 100 times older then they appear to be.
 * Played oddly in Tales of the Abyss.  is a clone, physically in late teens but in actuality only seven, with childlike emotional maturity to match the actual age. All the other characters seem to expect this trope to be in play, and react as if the character in question was evil or an idiot for not fitting their expectations grounded in this trope.

Western Animation

 * Jenny from My Life as a Teenage Robot. Justified in that she was designed to act like and have the emotional/mental capacity of a teenager, despite having been built only five years ago.