Some form of AppliedPhlebotinum results in one or more main characters regressing to an earlier age. It may be the result of a [[PhlebotinumBreakdown freak accident]], a plot on the part of the bad guys to incapacitate our heroes, or done on purpose so the characters can relive their childhood (at least temporarily).

For some reason this plot is often used in {{Fan Fic}}, where it's called a De-Aging story.

When someone dies in his rejuvenated state, expect him to [[ThisWasHisTrueForm turn back old]] immediately.

Contrast with ThirteenGoingOnThirty and PlotRelevantAgeUp. Compare ReallySevenHundredYearsOld.
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!!Examples:

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[[folder: Card Games]]
* Jhoira from the ''Magic: The Gathering'' storyline was exposed to timewarped water that dramatically extended her lifespan after a horrible accident at Urza's Academy. By the events of the Time Spiral storyline,she is over 1000 years old but looks like a 19-year-old--an almost literal fountain of youth.
** Of course, Magic has also had the literal ''card'' [[http://ww2.wizards.com/gatherer/CardDetails.aspx?id=1715 Fountain of Youth]] for the longest time.
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[[folder: Comic Books]]
* This was a favorite tactic of the extradimensional media mogul Mojo in the Claremont-era ''{{X-Men}}'' comics.
** Since time passes slowly in comic book universes, this is usually not an issue, as most characters' backstories aren't tied to a specific event. A notable exception is Magneto, a holocaust survivor canonically born in the 1920's. To get around this, he was reduced to infancy and then [[PlotRelevantAgeUp aged back to adulthood]] sometime in the 80's, putting his age firmly back in the traditional comic book "fucked if I know" category.
* TheDCU FifthWeekEvent ''Sins of Youth'' combined FountainOfYouth and ThirteenGoingOnThirty, with a combination of Chaos Magic and an "aging ray" turning all the teenage heroes into adults and all the adult heroes into kids.
* Happens twice from two different sources in ''{{Asterix}} and Obelix All at Sea'': First Obelix get de-aged as an after effect of drinking the Magic Potion, then our heroes discover the remnant of Atlantis, where everyone is a child (and happy from it) after having drunk from an actual FountainOfYouth.
* Three of ''TheSmurfs'' were child-ized when Peyo felt he needed a few child characters. There was no cure for the miniaturization, in a rare case where this trope isn't [[ResetButton reset]]. In fact, they didn't mind their new situation.
* Ra's Al Ghul's Lazarus pits. While not -exactly- the same thing, the pits are used at one point in ''BatmanBeyond'' to restore elderly Bruce Wayne back down to his 40's (as part of a Ra's plot to put his mind into Bruce's body).
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[[folder: Film]]
* The {{MacGuffin}} weapon of the BigBad in ''SkyHigh'' did exactly this.
* The titular character in ''TheCuriousCaseOfBenjaminButton'' has a similar problem: he ages backwards. He is born as an old man, and dies of old age as an infant.
* ''TheFountain'' involves a [[FramingDevice story within a story]] about a Conquistador's [[TheQuest quest]] for the Tree of Life, which can theoretically grant immortality. Outside the frame, there is a tree which essentially de-ages a cancer-ridden monkey and presumably grants immortality to the main character.
* This is the point of ''[=~17 Again~=]''.
* The SequelHook at the end of ''PiratesOfTheCarribbean: At World's End'' ends with (the immortality-seeking) Jack unfolding a map to something called the "Agua De Vida". The book that the next movie is based on revolves around the Fountain, so...
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[[folder: Literature]]
* In DianaWynneJones' ''House of Many Ways'', the grown wizard [[HowlsMovingCastle Howl]] became the DeliberatelyCuteChild Twinkle partly as a disguise and partly (he said) because he felt he deserved a better childhood than the one he'd actually had.
* TerryPratchett did this in ''Discworld/TheLastContinent'', combined with {{Thirteen Going On Thirty}}, when the wizards travel to a place (Fourecks), where, due to morphic instability, Ponder Stibbons becomes an eighty-year-old, and the Dean becomes a thirteen-year-old. And then there's [[IWasQuiteALooker Mrs. Whitlow...]]It wears off within a few minutes.
* Plot basis for ''TheCuriousCaseOfBenjaminButton''.
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[[folder: Live Action TV]]
* The ''{{Buffy}} the Vampire Slayer'' episode "Band Candy", in which all of the adults ingested chocolate bars that had spells inside of them that did not physically de-age the characters, but made them act as they had as teenagers. The episode is filled with hilarious examples of the Scoobies becoming more and more horrified by their parents, teachers, and even the stodgy Giles, as they act in irresponsible and (in the case of Giles) criminal ways.
* The ''StarTrekTheNextGeneration'' episode "Rascals", in which a transporter malfunction turns Picard, Keiko, Ro and Guinan into children, during which time the ''{{Enterprise}}'' is captured by hostile aliens. Despite the fact that they clearly keep their adult minds, they still have to save the day using childlike cleverness (communicator stuck to a toy car anyone?) rather than their usual methods.
* One of the few permanent examples comes from ''{{Kamen Rider Den-O}}'' which did this out of necessity. The actress who played female lead Hana quit unexpectedly in the middle of the series, but her character was too important to the story to write out. So the producers replaced the twentysomething actress with an eight-year-old. A subplot was added in which Hana was de-aged due to a time anomaly...and stayed that way. In a further subversion, none of the characters, including Hana herself, seemed to have much of a problem with this, and hardly any effort was made to find a cure.
* The ''Alien Rangers'' half-season of ''MightyMorphinPowerRangers'' was an extended plot-arc in which all the regular characters were age-regressed to young children.
* A mild example happens in ''StargateAtlantis'', when Sheppard's tenuous Wraith ally returned the years he had, um, borrowed, plus a bit. Possibly an attempt to balance out the six months he spent in a time dilation field.
** The Goa'uld Sarcophagi in ''StargateSG1'' also mildly extend life as well as healing wounds (but at the cost of negative psychological effects). However, this isn't that important to the Goa'uld, as they can just take another human host anyway.
* TheMightyBoosh: In the episode, "Fountain of Youth," Howard and Vince visit Naboo's home planet, Xooberon, in search of their own Fountain of Youth, but the Hitcher is looking for it, too. HilarityEnsues.
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[[folder: Manga & Anime]]
* Episode 9 of ''KeroroGunsou'' features HotShounenMom Aki Hinata getting turned into a teenager by Kururu's newest invention. In a later episode, the same gun turns Fuyuki into a kindergardener. even later, Natsumi, Keroro, Giroro, and Kururu also get zapped by this gun. [[spoiler: In one episode, the entire planet gets zapped by this gun, setting time back by about twenty years.]]
* ''SailorMoon'' has several examples. In the end of the third season (''Sailor Moon S''), Hotaru, after sacrificing herself to save the world, is brought back as an infant with no memories of her previous life. In the fourth season (''Sailor Moon [=SuperS=]''), Usagi and Chibiusa briefly exchange ages for one episode. In the beginning of the fifth season (''Sailor Moon Stars''), Hotaru [[PlotRelevantAgeUp rapidly grows back]] as her memories return, and the previous season's BigBad, Nehellenia, is returned to childhood to allow her to relive her life and avoid the mistakes that turned her into a villain in the first place.
* The Nehellenia story is reused in ''MermaidMelodyPichiPichiPitch'', but with Michal, the IllGirl and not-quite-villain.
* The ''[[RanmaOneHalf Ranma 1/2]]'' manga has the character Lukkosai, who tried to use the spring of drowned child to regain his youth, but instead just goes from way too old to way too young (both [[MiniatureSeniorCitizens about the same size]]). He then uses this form to get revenge on Happo while pretending to be his own grandson.
** Another story involved mushrooms that change your age in years to their length in centimeters. Ryoga eats a short one and then makes Ranma eat one. They spend most of the chapter trying to change back by growing mushrooms big enough to return them to their normal ages, while constantly sabotaging each others' mushrooms.
** And yet another story from the manga has Happosai feeling the pangs of old age. He finds out about a potion that will restore his youth, but first he needs the tears of a beast that is both male and female. The next thing you know, tear gas and onions fly liberally around Ranma. When he finally succeeds (by poking a pressure point that makes Ranma [[OcularGushers gush out tears like a firehose]]) he trips, and splashes the tears all over the floor. Kasumi wipes them up with a rag, but since it had been used to clean up soy sauce, the resulting potion regresses Happosai's ''mind'' to that of a baby.
* Alessi in Part 3 of ''JojosBizarreAdventure'' does this to opponents with his Stand, Sethan. The longer you stand in his shadow (through which Sethan manifests), the younger you get. Unfortunately for Alessi, when he tries this on Jotaro, he learns the hard way Jotaro was {{Badass}} even at age seven.
** It turns even worse out for him in the game (can't remember if this happened in the manga) when he used it on Joseph. Due to Joseph already being very old, it only regressed him into his late teens. Unfortunately for Alessi, that was the age when Joseph was at his prime and beat the Pillar Men, who were FAR stronger than any Stand users in Part 3. Joseph maintains his youth for a while after beating Alessi, but it eventually wears off and he returns to normal, wishing it could've been permenant.
* In ''KoiKoi7'', after Yayoi Asuka ([[EyepatchOfPower the one with the eyepatch]]) goes berserk, she's rebooted in one of Otome's spare bodies as a cute young girl named Gantai-chan who doesn't remember much of what happened before.
* In the Dark Tournament arc of ''YuYuHakusho'', an enemy uses a special mist meant to de-age it's victims past the point of conception. Unfortunately for him, his target was Kurama, and once his human body went into non-existence, his previous incarnation - full-powered demon Yokokurama - manifested and quickly dispatched him.
* ''RosarioToVampire'' has an example of this one. [[TokenLoli Yukari and Kokoa]], after realizing that they're not being taken seriously due to their age, use some of Yukari's age-up pills for a [[ThirteenGoingOnThirty temporary change]]. When the pills wear off, Kokoa finds herself ''younger'' than before. She finds herself in trouble shortly thereafter, only to be saved by the entire UnwantedHarem (sans Tsukune) turned into children along with her. Much cuteness and butt-kicking ensues.
* In ''DragonBallZ'', King Piccolo and Lord Slug used the titular {{MacGuffin}} to restore their youth, putting their power at prime. Roshi and his sister Baba actually drank from a Fountain of Youth, which, rather than restore their youth, fixed them at that age, unable to die from old age.
* Also, who can ever forget [=APTX-4869=] in ''DetectiveConan''? It regressed Shinichi Kudo and Shiho Miyano's ''body'' by around ten years, causing two [[TeenGenius Teen Geniuses]] to live in first graders' bodies. On the other hand, [=APTX-4869=] [[MillionToOneChance is lethal to other people]].
* In ''BlackCat'', [[BigBad Creed]] accidentally shoots Train with a Lucifer bullet, causing Train to transform back into a child (around 8 years old). It only lasts one episode (or a few chapters).
** As I recall, it was the [[MadScientist Doctor]] character who shot Train, or at least that was the case in the anime.
*** It was Creed in the manga. Doctor did it (on purpose) in the anime, but it ended up having the same effect in both versions.
* In ''MahouSenseiNegima'', WeaselMascot Chamo produces a jar of magical (and illegal) candy that can produce this effect as a sort of illusion (red candy [[ThirteenGoingOnThirty for older]], blue for this) both for fun on the part of the girls and as a StrangelyEffectiveDisguise when they [[ClearMyName become wanted people]]. Its been implied that TheMindIsAPlaythingOfTheBody, resulting in the aged-down characters acting with far lower mature than usual, though they retain their intelligence. This doesn't work both ways, so their [[TheCutie adult forms behave like giddy teens]].
* ''BlackLagoon'': a non-{{canon}} {{omake}} has the main cast regress to adolescence and, in [[BloodKnight Balalaika's]] case, beyond. For the record, [[TokenLoli loli-Balalaika]] is the cutest thing ever.
* [[AxisPowersHetalia England]], at one point, dresses up in a toga, wings, and halo, calls himself 'Britannia Angel', then, with a ''star-tipped fairy wand'', turns South Korea back into a baby for a strip. I am NotMakingThisUp.
** It's even become a meme in the fandom.
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[[folder: Video Games]]
* Gilgamesh from ''[[FateStayNight Fate/hollow ataraxia]]'' did this with a youth potion because, apparently, "this whole mess is stupid, screw you all". All the more hilarious, considering that he's been PresidentEvil (in the past), not to mention a [[KickTheDog dog-kicking]] KnightTemplar and BigBad who wanted to [[spoiler:unleash a terrible curse on humanity because only those who could survive it were worthly of being ruled over]]; when he reverts, he turns into a CuteShotaroBoy that everyone loves.
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[[folder: Web Comics]]
* When Mingmei in ''TheWotch'' discovers that she used to be Dr. Sorgaz, everyone assumes that she would want to be turned back... except the Sorgaz persona himself, who basically takes it as a second chance in life, and instead opts to merge with Mingmei permanently.
** Not to mention 21-year-old Evan's frequent temporary escapes into a 4-year-old ([[GenderBender female]]) body, called "Lily".
** Seriously, what about the entirety of ''Adventures in Babysitting''? Even though Evan turns into an adult female, Anne, Robin (Robyn?), and Jason were turned into children (Okay, they turned into the opposite gender too, but that hardly matters!)
* Emily from ''{{Misfile}}'' suffers from this, though she was only regressed by two years rather than the usually much larger amount. Since she was just about to turn 18 and had received her acceptance letter from Harvard these were a rather vital couple of years though.
* In one strip of ''AxisPowersHetalia'' England de-ages Korea to make him less annoying.
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[[folder: Web Original]]
* Happens to both Marie and Heather in ''[[{{ErikasNewPerfume}} Erika's New Perfume]]'', each reduced to three years old. Neither returns to their proper age.
* ''WhateleyUniverse'' example: retired U.S. Navy officer Sam Everheart was injured trying to save a HotScientist from bad guys. The 'hive' of nanites infected him and saved him.. by turning him into someone a lot younger. He now looks like his deceased 17-year-old daughter.
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[[folder: Western Animation]]
* ''TheEmperorsNewSchool'' features the actual Fountain of Youth in one episode. Yzma uses it to become a (surprisingly attractive) teenager in order to trick Kuzco. It also features Pacha and Chica becoming teenagers, and ends with Yzma and Kuzco becoming (somewhat freaky looking) babies.
* One episode of ''LiloAndStitch'' involved an experiment known as the "Baby-Fier" that turned its victims into infants.
* ''SpiderManTheAnimatedSeries'': The [[EvilOldFolks aged crime lord]] Silvermane's long-standing goal was to find a way to restore his youth. Unfortunately, the AppliedPhlebotinum worked a little too well, and he reverted to infancy. A later attempt to fix it ''also'' works too well, restoring him to his original old age instead of to his target age.
* One ''StarTrekTheAnimatedSeries'' episode had the Enterprise get sucked into a parallel universe where entropy was reversed and things aged backwards; they quickly started getting younger and younger, leading to an adorable 7-year-old Spock trying to hold the mission together.
* Two episodes of the original ''[[sternAnimation/{{Teenage Mutant Ninja Turtles 2003}} Teenage Mutant Ninja Turtles]]'' dealt with this in "Back To The Egg" it happened to Leonardo and Michaelangelo, and in "Adventures In Turtle Sitting" Leonardo, Raphael, and Donatello turned into children and the slacker of the group Michaelangelo had to look after them.
* ''CodeNameKidsNextDoor'' had an episode about it. The Delightful Kids from Down the Lane wanted to destroy it.
* Peach got turned into a baby in ''SuperMarioBros'': Super Show after falling in The Fountain of Youth. To get her back to her real age, the brothers had to [[{{reverse polarity}} make the waterfall in the fountain go up instead of down and put her in the fountain]].
* In the ''DuckTales'' episode "Sweet Duck of Youth," Scrooge looks for the literal Fountain of Youth so that he can live longer. It turned out, though, that the fountain only made a person ''look'' younger when reflected in its waters.
* In the ''JusticeLeague Unlimited'' episode "Kid Stuff", Morgan Le Fay's son Mordred gets his hands on a powerful amulet and, in a fit of pique after his mother insists he's too young to be responsible for it, banishes all the adults in the world to a pocket dimension. Morgan gets around this by turning some of the Justice Leaguers (Batman, Superman, Wonder Woman, and Green Lantern) into 8-year-old children so they can get the amulet back.
* ''{{Ben 10}}'' also features the literal Fountain of Youth (though it's more of a natural pool or spring). Max turns into a 10 year old and is revealed to be similar to Ben at that age. Ben and the aliens turn into toddlers. It wears off.
* In SpongebobSquarepants, Plankton develops both a super-aging ray and a de-aging ray. The latter ends up working a bit too well, turning him into an even smaller protozoa.
*''{{Futurama}}'' did this in "Teenage Mutant Leela's Hurdles", where overexposure to rejuvenating tar resulted in Professor Farnsworth going back to late middle age, and the rest of the Planet Express crew (including the robot Bender) becoming teenagers. An attempt to reverse the process made things worse, and everyone but Leela (who opted out of the cure, in order to relive the childhood she never had) started regressing into early childhood.
* An old episode of ''{{The Care Bears}}'' has this. One of the villians ends up turned into an infant and her partner has to watch after her.
* In an episode of ''Hercules: The Animated Series'', Pain and Panic try to kill Hercules (again), this time by using water from a spring that reverses aging. They accidently get water on not only Hercules, but on Icarus, Adonis, Pegasus, and Pain himself. Panic and Cassandra have to watch them since they're all infants until they can find a cure.
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