Overnight Age-Up

The reverse of Fountain of Youth, this is a scenario where Applied Phlebotinum (frequently in the form of a wish) turns one or more children into adults. Hilarity Ensues, and the kids either fumble around awkwardly in their post-pubescent bodies, or learn An Aesop about adult responsibilities and/or not being in too big a hurry to grow up.

Can also include a young adult character or one in early middle age becoming elderly.

Not to be confused with Plot-Relevant Age-Up or Soap Opera Rapid Aging Syndrome. Sometimes combined with Older Alter Ego or She's All Grown Up.

Anime and Manga

 * Episode 18 of Keroro Gunsou revolves around Natsumi being subject to a Transformation Ray that turns her into an adult. She's even more annoyed when she finds it's part of a scheme on Keroro's part to enter her in a beauty pageant/manzai contest and win a rare model kit.
 * In the manga, it's just a straight-up beauty contest, in which Natsumi experiences a Wardrobe Malfunction as she reverts back to her original age—all in front of horny geeks.
 * A later episode has the Bipolar Momoka experience this same growth, at the same beach, but there is no contest for her to partake in. Instead, it's some sort of Iron Man competition.
 * Another manga-only story has Natsumi being hit with the same ray at a ski resort, being hit on by the same creeps from the beach, and another wardrobe malfunction, courtesy of explosives.
 * A wish of this kind that comes true forms the premise of Asatte no Houkou—and turns a nearby adult into a kid as a side effect.
 * In Mahou Sensei Negima, Negi's Non-Human Sidekick Chamo produces a jar of candies that alter one's age (specifically, the red ones for older, blue ones for younger). Of course, when he uses them, several of his students notice that He is All Grown Up without actually recognizing him.
 * Apparently they're based on a spell Evangeline invented to make up for her body's age. Once the gang gets more friendly with her, she provides them apparently for free (the knock-offs are expensive).
 * A similar idea to the one above formed the premise of Marvelous Melmo.
 * The anime series Hime-chan no Ribon (Hime-chan's Ribbon) is largely based on this, as Himeko frequently uses the magic ribbon to appear as an adult.
 * Guu from the anime Haré+Guu can appear as an adult when she needs to. Since she normally appears to be the same age as ten year old Haré, this counts. Sorta. It's quite possible neither age is her true form.
 * Vivio turns into an adult in Magical Girl Lyrical Nanoha Striker S
 * And in the sequel manga, Magical Girl Lyrical Nanoha Vi Vid, Vivio is given her own Intelligent Device which allows her to do this back and forth voluntarily. As is par for the course in this series, Vivio is very mature about it and doesn't use the power for petty and immature things.
 * Lambo from Katekyo Hitman Reborn! does this in spades with his Ten-Year Bazooka, turning him from a five-year-old Bratty Half-Pint into a suave 15-year-old Bishonen. However, the transformations only last for about five minutes. I-Pin has been hit by the bazooka more than once as well, and in one notable misfire, Gokudera actually became younger after accidentally being hit.
 * In Ah! My Goddess, Skuld gets to be an adult for a day due to Phlebotinum Breakdown. Her older sister, Urd, goes the other way. Despite being desperate to return to their original forms, they find ways to make the best of it.
 * While not a true instance of this trope, it's played out exactly the same. An episode of Zettai Karen Children has Minamoto becoming hypnotized into seeing the three young girls he watches over as five years older—and much more irresistible—teens. This was to test if he really might be falling for them.
 * In one episode of Sailor Moon SuperS, PallaPalla switches Sailor Moon and Sailor Chibi Moon's ages around as a joke. A similar plot is present in the original manga.
 * Inverted in Nanaka 6/17, where, thanks to Trauma-Induced Amnesia, 17-year-old Nanaka believes that she's actually six-years-old and was magically transformed into an adult.
 * In Detective Conan OVA: The Stranger in 10 Years, Haibara finished an antidote drug to make Conan/Shinichi 17 again but he was sick that time. Once he took the drug, he didn't turn back into a 17 year old Shinichi,
 * Instant Teen: Just Add Nuts; after all, there's a reason Tokyo Pop released the English translation under that name, and that reason is this trope being a major plot element.

Comics

 * The DCU Fifth Week Event Sins of Youth combined Fountain of Youth with this, with a combination of Chaos Magic and an "aging ray" turning all the teenage heroes into adults and all the adult heroes into kids.
 * Of course, we also have to mention Captain Marvel (NOT Shazam), a boy who transforms into an adult superhero. This actually causes a problem when he gets involved with a superheroine his own age, and they have to break up because their teammates, who don't know Marvel's civilian identity, would take it the wrong way.
 * That may have been the origin of Major Might's very brief affair with Amazonia in Love and Capes.
 * Subversion: Darkdevil from Spider-Girl, according to his origin and mind-trip sequence. Subverted in that he's had to live as a twentysomething for a while and seems to prefer it that way.
 * Was once used by Mojo on the New Mutants. It also turned them evil.

Films

 * Probably the most famous film example of this trope is the 1988 Tom Hanks movie Big.
 * Also, the former Trope Namer, 13 Going on 30.
 * Also the movie Freaky Friday, though that's more of a body switch deal.
 * Speaking of which, there were also 80's classics like Vice Versa and Like Father Like Son.
 * And Summer Switch.
 * Another one from 1988 - the Made for TV Movie Fourteen Going On Thirty.
 * Italian film Da grande, which actually predates Hanks' Big.
 * From Kirikou and The Sorceress
 * Adam Sandler's Click uses a ratcheting variation, to familiar Aesopic ends.

Literature

 * In the children's book Magic by the Lake, the protagonists are four siblings (three girls, one boy). Two of the girls wish themselves into teenagers and go on a double date, and their brother and sister have to chase them down and undo the wish before the date goes too far.
 * In Freaky Friday, a mother and daughter change places for a day, giving us a Fountain of Youth and a 13 Going on 30 in the same story.
 * In the sequel, set several years later, the father and brother also trade places (sequel to the book, not the film).
 * In Flossie Teacake's Fur Coat, the eponymous 10 year old longs for the fun and independence of her older brother and sister. When she puts on her sister's fur coat and does up the magical third button, she becomes the 18 year old Floz.
 * There's a boys' version by the same author, Hunter Davies, entitled Come on Ossie, featuring a boy called Oswald who turns into an 18-year-old when he borrows his grandfather's medal. There are several sequels to both books.
 * In Five Children and It by E. Nesbit the older four get annoyed with how they have to chase their baby brother around all the time, so they wish him into a grownup. Unfortunately, he didn't learn all of the lessons associated with growing up, such as not being a total prat, and turns out to be even more annoying this way.
 * In Howl's Moving Castle by Diana Wynne Jones, Sophie is cursed by a witch and turned into an old woman.
 * In The Thief Lord, there is a magical merry-go-round that can act as either a Fountain of Youth for those who ride it, or provide this effect, depending on what animals you ride. At the end of the book, one character chooses to age himself from a young teenager to a young adult - and is stuck that way.
 * In Replica #16, Happy Birthday, Dear Amy, Amy, Number Seven mysteriously ages into her twenties overnight on her thirteenth birthday.
 * Jake in the Animorphs book The Familiar. He wakes up as a young adult in a world where the Yeerks control the Earth. By the end of the book, he's back to being a young teen, though.

Live Action TV

 * First used in the Star Trek universe in the original series episode "The Deadly Years" (Ep. 2.12) in which the Enterprise crew discover a planet where the colonists are afflicted with a rapid aging syndrome, and are affected by it themselves. Star Trek: The Next Generation does the "adults becoming elderly" version a few times:
 * In the season two episode "Unnatural Selection", the Enterprise receives a distress call from the USS Lantree, where they find everyone dead from rapid aging, and must find the cause before scientists on a research colony suffer the same fate.
 * Season six episode "Man of the People" has Deanna aging as a side-effect of a psychic ambassador using her energies to influence the outcome of his mission.
 * The DS9 ep "Time's Orphan" has Molly O'Brien aging from 8 to 18 through a Negative Space Wedgie.
 * Sabrina The Animated Series used it in one episode.
 * "Brief Candle," from season one of Stargate SG-1, finds Jack O'Neill infected by Goa'uld nanites which send him flying into old age.
 * The TV Show Big John Little John was half this trope and half Fountain of Youth as the titular character switched back and forth between childhood and adulthood at least once an episode.
 * Isabelle from The 4400 goes from being an infant to being twenty-something over the course of an end-of-season montage. She turns out to be super-intelligent but naive. She begins a relationship with Shawn, which her father finds out about. Father is outraged, taking the view that she is not legitimately an adult.
 * In Power Rangers Dino Thunder, Cassidy, Devin, and other citizens of Reefside (as well as a pop star) were turned elderly by an youth stealing vacuum monster.
 * A donkey-pomegranate-vacuum monster!
 * In Power Rangers Zeo, Billy got turned into an old man for two episodes (his last; old Billy is The Nth Doctor being used to cover for the actor saying Screw This, I'm Outta Here to behind-the-scenes Jerkassitude.)
 * In the Gilligan's Island episode "Meet the Meteor" (Ep.2.32), a meteor lands on the island and emits cosmic rays, causing all of the characters to age rapidly.
 * The fifth season Supernatural episode "The Curious Case of Dean Winchester" involves a poker game played using years as stakes; Dean doesn't fare very well in the game and ages rapidly. He gets better.
 * This is the main power of the Old Dopant in Kamen Rider Double.

Web Comics

 * In a storyline in Kevin and Kell, Coney and Nigel use the time machine to age themselves from babies to adults so they can spend one day as adults before Nigel moves away. A funny moment is when Coney spits a whole bunch of teeth and says "Baby teeth".

Web Original

 * In this Deviant ART animation, a girl ages up, presumably by trading ages with the guy opposite her who ages down.

Video Games

 * Played straight in the video game series The Sims. The characters "poof" from one age group to another once the requisite time has passed. In the first game this only applies to babies becoming children - children never become adults and adults never grow old - but from the second onwards it affects everyone.
 * The Old status effect in Final Fantasy V continuously lowers the affected character's level, weakening them and turning their hair white. Strangely enough, it also affects Cool Old Guy Galuf.
 * The Legend of Zelda: Ocarina of Time to an extent. Link goes to sleep for seven years, aging gradually, but it's instantaneous to him and the player.
 * Getting hit by a Magician's spell without armor in Ghouls 'n Ghosts turns Arthur into an old man.

Western Animation

 * Lilo and Stitch, "Skip": Tired of being treated like a kid, Lilo uses an experiment that lets her skip ahead 10 years at a time to become a teenager, then a grown woman. Unfortunately, she finds that she's been gone all those years, and after the second time-skip, bad guy Dr. Hamsterviel has managed to take over the Earth.
 * Used also by Numbuh Two in Kids Next Door, who has a crush with Numbuh Five's big (evil) sister and repeatedly ages himself so he can get close to her.
 * Numbuh One was turned into an adult once by the Delightful Children in a particulary sad example: all operatives must be decomissioned when they turn 13, and not being a child anymore means that One must leave the KND.
 * Three episodes of Dexter's Laboratory deal with the mental mechanics. In one, he wishes to stay up late to watch scary movies, so makes himself older. But he overshoots, becomes elderly, and is unable to stay awake to watch. In the second, he is too small in size to ride amusement park attractions, and ends up giving himself a case of size change instead. And in the third, he becomes a teenager... but instead of becoming a handsome lad, he becomes an absolute Nerd with pimples and crooked teeth.
 * American Dragon: Jake Long: "Young At Heart."
 * In The Fairly OddParents episode "The Big Problem!", Timmy wishes he were grown up, then almost loses his godparents, who can only grant wishes to children.
 * In SpongeBob SquarePants, Plankton once developed an instantaneous aging ray.
 * Rocko's Modern Life. Rocko, Heifer, and Heifer's Grandfather are on a cruise filled with senior citizens when they cross The Bermuda Triangle, which alters their ages. Giving us a mix of Fountain of Youth and this trope, all the elderly revert back to young(er) adulthood, the ship's crew become babies, and Rocko and Heifer become elderly.