Fountains of Youth in Fan Works.
Crossovers
- In the second part of Alliance of Amazons and Izuku
, Recovery Girl is attacked by a villain with a de-aging Quirk who tries to age her out of existence. Luckily for her, he underestimates her age, and ends up turning her back to her 20s. - The God of Destruction comes to Remnant:
- In a way, this happens to Rexy. In Jurassic World, she was in her twenties, making her elderly by T-Rex standards. Then she is transported to the world of Remnant and transformed into a Faunus woman, still in her twenties and now a smoking hot Statuesque Stunner.
- Gojira is really old, but his new body looks like a teenager.
- Leo and Junior should be at least in their twenties, and Minya should be far older than that, but when they arrive in Remnant, they turn into children.
- Kyon: Big Damn Hero: Nagato provides this function for Kyon, Mikuru, and Kanae. Otherwise, the Year Inside, Hour Outside training would make them age by over a year every night.
- RWBY: Epic of Remnant: Gudako and her Servants get transported to Remnant. To their chagrin, they have been turned back into teenagers.
- Wilted Flowers: The akuma Princess Justice can 'rewind' anyone struck by their powers, erasing any memories they have of their lost years in the process. Most of their victims are teenagers who get turned back into toddlers; adults are turned into teenagers.
- In Slice of Heaven, Rover accidentally de-ages Nadia into a child. He hasn't seen humans in a long time, so he goes off his memories of children when changing her looks.
- This happens to all of the main characters at various times in the Avantasia Protag AU series due usually to Aaron's invention research accidents. There are two "types". The most common is when the character de-ages into a mini version of themselves as they are currently, complete with recognizing the others and retaining their personalities and relationships (although child-ified i.e. clinging to their (when adult) romantic partner a lot and wanting to be held). The second, less common, type is when the character becomes who they actually were as a child, seeing the other characters as strangers and showing personality traits they might've grown out of and can appear quite different from the first type.
- The Facing the Future Series features a story where Danny and Sam revert to five-year-olds in homage to a Star Trek: The Next Generation episode entitled "Rascals."
- In Wearing Skin Too Small, Pico is turned into a child after making a deal with Daddy Dearest.
- Happens to House and Wilson in Monkey's Paw Syndrome due to magic. Makes Sense In Context.
- Jonathan Joestar, The First JoJo: Jonathan, courtesy of a surprise attack from Alessi, is reverted to his 6-year-old self. It lasts for most of chapter 9 before it's finally reversed.
- Loki turns the Avengers and himself accidentally into toddlers in this story. Unlike most others in this genre, the children retain their memories and powers, leading to more than the usual Badly Battered Babysitter plot.
- The main premise of Karma in Retrograde is that Dabi gets reverted to his sixteen-year-old self as Touya Todoroki with none of his memories in the past five years, leaving him an innocent, but troubled U.A. student.
- In My Hero Academia: Mechanical green, after saving Eri from the Shie Hassaikai, Stain and Inko are accidentally hit by "Rewind" and de-aged, Stain now roughly being 15, and Inko being roughly a decade younger. Stain likewise regained his nose and, thanks to his now younger age, was able to be accepted into Nezu's villain rehabilitation program. Inko meanwhile lost the weight and regained the height she lost from the years of stress following Izumi's diagnosis as Quirkless.
- Characters getting hit with age regression quirks is a common fic premise in the fandom in general. Archive of Our Own has a whole category of them.
My Little Pony: Friendship Is Magic
- Best Friends Forever features Twilight having a breakdown after Fluttershy dies of old age and regressing the Mane Six to infancy in a desperate attempt to make them as immortal as her. When Celestia finally catches up with Twilight, it's heavily implied that this process has been repeated multiple times over the course of centuries.
- Twilight accidentally turns herself into a filly in Diaries of a Madman, after one of her spells backfires.
- In Falling Backwards Rainbow Dash receives a serious brain injury that leaves her mentally a child who Twilight is forced to care for. Unfortunately, there seems to be no way of curing her, so in as a desperate measure, Celestia uses an age spell to regresses her back into a child, so she at least has a chance of growing back into her old self.
- Getting Back on Your Hooves: This is what Poison Joke does to Trixie. Interestingly, it also affects her mentally (and since she didn't have a happy childhood...)
- The Manehattanverse story "Fillie's Play" focuses on this, as a magical mishap during one of Twilight's experiments turns her and the rest of the Manehattan 6 back into fillies. And has the opposite effect on Orange Sherbet's daughter, Tangerine.
- The Youth Restoration Spell from My Little Alicorn reverts any pony it's used on to any prior age for two hours, after which they are supposed to turn back to normal. Naturally, Luna thought it would be a great idea to use it on Celestia as a prank, not realizing that it works a little bit differently on Alicorns.
- Pony POV Series:
- When Princess Gaia tries to take over the world and turn it into a utopia, she de-ages the citizens into foals, thinking they would be happier that way. Princess Celestia was horrified and feared that the pregnant mothers' unborn children had been erased. Fortunately, they weren't, and once the mothers were returned to normal, their pregnant bellies returned.
- The fragments of the Rainbow of Light can do this to their holders, but not forever. Patch used them to stay young for a really long time. The elderly dragon Basil used one to return to his physical prime while keeping his power boost from his extreme age.
- In the Dark World Timeline, when Celestia and Luna are freed from their statue state... they've been turned into foals, with Celestia having forgotten her life as an adult in the process from having lived a 1000 years in an idealized mental world made by Discord as a gift. Luna's mind is the same and she spent the thousand years astral projecting.
- In The Two Sides of Daring Do, spells and potions that can alter your age are mentioned, but they do not actually affect your lifespan. For example, if an old pony became young again, he would still eventually die of old age in a few years. In one of the endings, Daring Do chooses to start life over as a filly. Since she was technically only a few days old, she still had a full lifespan ahead of her.
- Little Uzumaki: Konoha has one; notably, they don't bother keeping its existence a secret, as it regresses whomever uses it mentally to match whatever physical age they end up at. Unfortunately, Kakashi accidentally takes Team Seven there for a rest stop, leading to all of his genin accidentally turning into two-year-old toddlers right before the Chuunin Exams.
- Sakura has accidentally drunk from the spring four times; originally, she was a contemporary of the legendary Sannin, with Tsunade as her best friend. Her aging parents are heartbroken to learn she's reverted yet again.
- In Ranma Restart: Bid for Freedom, Cologne uses the newly-created Spring of Ranma to make herself young again so that she can serve as the template for the new Spring of Drowned Girl.
- Changes, where Egon of ''is turned into a child due to an experiment gone awry. The nightmare sequence where he de-ages into nothingness while screaming for help from his uncomprehending teammates is chilling.
- With Strings Attached: The four awaken to find themselves on another planet and sixteen years younger — their 1980 minds in 1964-age bodies. They think they've been genuinely de-aged, but, at the end, they find out that they're in cloned bodies. The Fans wanted to give them fresh new bodies as payment for going through all the hassle on the planet; they were going to age the bodies, give them appropriate scars, etc., after the adventure was over. However, things got out of hand, and a whole lot of magic got dumped on the clone bodies. Because the Earth universe has no magic, this meant that the four had to be transferred back to their original bodies in order to return home.
- Insontis and Insontis II detail this happening to Kirk and Spock, respectively.
- Happens to Data in Small Packages by RebeccaOTool, via an unoccupied android body left by his late father. Played for laughs. At first.
- Star Wars: Galactic Folklore and Mythology: Bothan myths claim that a certain island holds a magical lake that will reflect even the oldest people as they were in the prime of their youth and, if drunk from, will grant eternal life — for the pure of heart. Depending on the version, undeserving people who drink from it will either die on the spot or turn into toad-like beasts cursed to stand eternal vigil at the lake.
- This fanart has Fumi, having turned back into her 5-year-old self, being consoled with a (sweet blue) flower by the present-day Akira.
- In the second story of the Gensokyo 20XX series, we have this happen to Reimu, due to the side effects of a hibernation spell Ran cast on her. This also results in a Mode Lock, as she stays like that until the final chapter of 20XXV, where it's implied she's full grown.
- In The Decepticons Revenge, the Decepticons use a special gun to turn Elita One, Chromia, Ironhide, Ratchet and Skyfire into sparklings. Starscream later uses the gun on himself.
- In Second Sunrise, a malfunctioning weapon turns several of theAutobot frontliners into sparklings.
- Kitsune: Conner Jeremy Fox, a 39+-year-old man, age-regressed into a teenage girl.
- Ma'at: 70+-year-old Doctor Daniel Edward O'Neill transforms into Dani, a 20-something female priestess of Ma'at.
- The Accidental Warlord And His Pack: Jaskier is turned into his child self in a magical accident.
