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 Is All Grown Up.
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- A commercial for GoodNites shows a young boy waking up several years older, his pyjamas having been ripped up due to his increase in size.
Anime & 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!, in a large Shout-Out to Marvelous Melmo, 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).
- As partially mentioned above, the entire premise of Marvelous Melmo is based around age-changing pills: the titular character, a nine-year old girl, can use some of those to age herself into a nineteen-year old girl or an old lady, as well as turn into a baby.
- The anime series Himechan 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.
- In Rosario + Vampire the Token Mini-Moe wishes she had the Most Common Superpower the others in the Unwanted Harem has, so she comes up with this magical spell...
- Vivio turns into an adult in Magical Girl Lyrical Nanoha Striker S when the Cradle's defense system forces her to battle Nanoha. She returns to her original age once the Relic inside her is destroyed.
- And in the sequel manga, Magical Girl Lyrical Nanoha ViVid, 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 Bishōnen. In one instance, he does it twice during a battle and turns 25 — and is a veritable powerhouse, to boot. 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. One can only imagine the utter hilarity that occurred during the five minutes the adult Gokudera where his younger self used to be.
- 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, he's a 17 year old Conan Edogawa warped 10 years in the future. Thankfully, it ends up being a dream.
- 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.
- In One Piece, this is one of the abilities granted by Jewelry Bonney's Devil Fruit. She prefers to make her opponents younger, though.
- 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 twenty-something for a while and seems to prefer it that way.
- Was once used by Mojo on the New Mutants. It also turned them evil.
- In Green Lantern, teenage Green Lantern Arisa's power ring responded to her desire to be older and aged her into a twenty-something literally overnight.
- The My Little Pony: Friendship Is Magic fanfic Grow Up, Apple Bloom has the titular filly acquire an aging potion from Zecora, which turns her into a mare almost as old as her sister Applejack.
- Another MLP example, Inner Demons: Queen!Twilight Sparkle magically ages up the Crusaders, and also offers to give them their cutie marks, in exchange for their loyalty. Sweetie Belle and Scootaloo take her up on it, while Apple Bloom refuses. This gets reversed at the end of the story, with help from the Princesses.
- Yet another MLP example — at the climax of the Dashverse story Hot Heads, Cold Hearts, and Nerves of Steel, Sombra's attempt to steal the lifeforce of the kidnapped foals to empower himself causes them to physically age up at least ten years each, though their mental state stays the same. This gets reversed after Sombra is defeated.
- A Shadow Of The Titans: In one chapter, Jade uses a staff based on Mad Mod's technology to drain Cyborg's age, making herself an adult while turning Cyborg back into a child. Uniquely, she's the one who reverses the process, as she's sick of the Power Incontinence that came with the change.
Films — Animation
- From Kirikou and the Sorceress Karaba, the Sorceress, kisses Kirikou and he goes from a cute boy to a handsome man in a matter of seconds.
Films — Live-Action
- 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 (2003), 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.
- And, of course, the original Freaky Friday
- Another one from 1988 - the Made-for-TV Movie Fourteen Going On Thirty.
- Italian film Da grande, which actually predates Hanks' Big.
- Adam Sandler's Click uses a ratcheting variation, to familiar Aesopic ends.
- Cirque Du Soleil Journey Of Man uses this in conjunction with Fountain of Youth. When Love Redeems the middle-aged Everyman and he accepts his childhood bowler hat, he magically ages into an elderly man, but having realized true happiness and a new vigor for life, he sees the transformation as making him "young again".
- In a definitely non-humorous example the titular Warlock curses a young woman to age by 20 years in a night, every night for no reason other than she got into his way.
- 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 an Overnight Age Up 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. The rapid aging was caused not by Amy's unusual genetics, but by some sort of implant, and she reverts to being a teen when it is destroyed.
- 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.
- 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.
- 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. When the Old Memory is destroyed, the effects are reversed.
- In a storyline in Kevin & 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".
- These downloadables (with pay) from DreamTales Comics.
- Marcy and Francis in PvP both undergo instant age-ups in response to certain events (having sex for the first time, Francis deciding to leave home when the office relocates to Seattle, etc.) They instantly "level up" into their new ages, complete with a display of their changed stats. It gets lampshaded by other cast members.
- 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.
- The Age-O-Matic spray in Tomodachi Life turns kid Miis into grown-ups. There's also the oposite effect in the Kid-O-Matic spray.
- Lilo & Stitch: The Series, "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.
- 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 thanks to Dee Dee's antics, 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
- This was the premise of the Rugrats special 'All Growed Up' that spawned the spinoff series All Grown Up!.