- Axis Powers Hetalia fanfic Gankona, Unnachgiebig, Unità: Like in canon, all the countries are this. Germany, Italy, and Japan all look 20 despite them being 1054, 2484, and 3284 years old respectively. That, and they are also really hard to kill.
- A fandom example: for a while, it was vaguely popular among Harry Potter Fan Fic writers to claim that (a few hours per weekday for a year of) Time Travel had sufficiently aged Hermione that it is no longer squicky to have her paired with Snape. Which is silly on the face of it, since two or three extra hours every weekday for an entire year (without even considering summer vacation) adds up to less than thirty days of additional aging. Not to mention she was petrified in her second year for about the same length of time. That's right folks, she broke even.
- In Power Girl story A Force of Four, Power Girl is 69 and looks 29. There's delayed growing and aging involved.
Superman had called this city home since 1938. Now it was 1985, and whatever responsibilities it held for superhumans had been passed on to her own white-clad, red-caped shoulders. Some people below saw her, pointed, shouted at her, waved their hands wildly. She gave a half-hearted wave back, not really looking at them.
Power Girl wore a white leotard, cut high on the thighs and low on the chest... a great distraction for evildoers, and any other males past puberty. Her feet were hidden in blue buccaneer boots, she wore blue gloves (none of that stupid bare-handed stuff for her; Kal's leaving fingerprints had gotten him in trouble more than once), and had a red belt with a gold buckle slung low on her hips (she had it and wasn't afraid to show it).
She had been born in 1916, and had the body of a 29-year-old. Her origin was far, far stranger than Kal's.
- Child of the Storm has pretty much all Asgardians as this. Of humans, examples include Doctor Strange (over 400 actually 500,000, doesn't look a day over 40), his former apprentice Wanda Maximoff (45, looks under 30), Natasha (over 80, looks under 30 thanks to the Russian Infinity Formula), and Bucky Barnes, the Winter Soldier, thanks to the Russian Infinity Formula and Zola's modifications. There's also the First Class of X-Men, who were de-aged during a Noodle Incident some time before the story starts, leaving them all looking considerably younger than they are (Sean, for instance, is in his 60s but looks in his 30s).
- Maoimi in Soulless Shell is supposed to be seventeen, but her age isn't mentioned until the final chapter and thanks to the hilariously clumsy writing she comes across as being about seven. The MSTing does not let this go un-riffed.
Romsca: (as Maoimi) C'n I 'ave some candy, pwease, nice mister paedophile?
- Zigzagged with in Kyon: Big Damn Hero, with Kyon, Mikuru, Kanae, and Yuki aging a year during training every night, but having Yuki de-age them afterwards with added Laser-Guided Amnesia. Then, Kyon time-travels all over the place, running errands for his future self, which makes things more complicated.
- The Naruto fanfic Hakumei has a jounin-level villain in her late teens convincingly disguise herself as a 12-year old genin.
- In Frozen fic What About Witch Queen?, OC Kai Madsen is said to look like a teenager despite being about twenty-five. His father, in contrast, is younger than he looks.
- With Strings Attached takes place in 1980; when The Beatles are sent on their journey, they are physically (but not mentally) de-aged from the thirty-something men they are into the ages they were when Beatlemania was at its height.
- In The Keys Stand Alone: The Soft World, there are several instances of the four being considered “callow youths,” which makes some people seriously underestimate them. Which they use to their advantage whenever they can.
- Lady Venus in The Blue Blur of Termina. According to Tatl, despite appearing to be in the human equivalent of her late twenties, she is actually over five hundred years old.
- The Chiss Force user Grein in Star Wars Paranormalities is biologically 41 years old, but doesn't look much older than 30. Lampshaded by other characters and Hand Waved by her as being a combination of a Force-user and a Chiss. And that isn't even taking her chronological age into account.
- Happens with Loki in this fic, where he decides to use his new appearance to hide from the alien army he pissed off previously.
- Discussed by Empath and Homnibus in the Empath: The Luckiest Smurf story "The Exile Of Psychelia", where the elderly human wizard is surprised by how young Empath looks for 151 years of age.
- In Bait and Switch Eleya describes Admiral Amnell Kree as "well-preserved but something about her leads me to peg her age as sixty-something." And that's just the Trill host we're talking about; the symbiont lands in Really 700 Years Old territory.
- Cole in Bringing Me To Life looks twenty but is over seventy-years old. Being an A.I. in The Matrix he doesn't age physically. Cole's adopted father, Smith, also an A.I. is over six hundred years old, but only looks like he's thirty or forty.
- In the Gensokyo 20XX series, we have this with Reimu, who was age-regressed to a child during the events of 20XXI, and Yukari, who fits into youkai tradition by being older than she appears.
- Syaoran in Shatterheart looks fifteen but is actually twenty-one. He didn't physically age when he was imprisoned for seven years.
- Hivefled's Grand Highblood, as a purple-blood troll, would have lived potentially thousands of years anyway, but after a life-extending kiss from Condesce he's lasted much longer than he should have. An elderly seatroll, whose caste usually live even longer than the purples, notes that when he was born GH was already an adult in his prime.
- Saito Hiraga in Soldier of Zero has a note in his file that says while he looks to be about seventeen, he's closer to thirty. He even mentions that his younger sister is married and expecting.
- In Pokémon Reset Bloodlines Lt. Surge looks around 30 but is stated as being a war veteran. Given that the last war ended 40 years ago, he's at least 58.
- My lesbian life with Monster Girls: Monster Yurisume: Mara looks to be fourteen, but is in fact twenty years old by small-breed arachne standards. In fact, according to her, her sisters think she actually looks older.
- In The Second Try, Shinji and Asuka look fourteen but they're in their early twenties.
- Once More with Feeling: After travelling to the past, Shinji looks (naturally) like a teenager. However he's mentally and emotionally way older. A secondary character notes this.
- Queen of All Oni: Blankman, Jade's Enigmatic Minion, looks middle aged despite the fact that a flashback to his childhood is set in 1929.
- Played for laughs in a The Martian fic in which Dr. Beck admits that he got a lot of Doogie Howser jokes while in med school and couldn't buy alcohol until he was 23 because he looked considerably younger than he actually was and no one would believe his ID was real.
- In Out of the Big Black, Jim Kirk has become this trope, having experienced sixty years of life and skipped eighty-nine more, before being physically de-aged back to around thirty.
- In The Bridge multiple individuals have very slowed or stopped aging once they hit their physical prime.
- Princess Celestia is over a thousand years old and yet looks like she's in her late 20s, early 30s at most; on account of her being an alicorn and creation of Harmony.
- Anguirus, an artificially created elemental kaiju, is over 70,000 years old and hardly looks any older than Celestia.
- Monster X is roughly 400 years old but looks like he's in his mid 20s when in human form, likely on account of his aging being halted when he changes from a normal Xilian to a kaijin.
- Princess Cadance is actually Princess Amore's daughter, having been spirited away by King Caesar and forward through time to about two decades before the start of the show. This makes her over a thousand years old and just a bit younger than Celestia and Luna, yet she is physically in her mid 20s.
- In the Naruto fanfic Sugar Plums the protagonist Ume is on average thought to be three to four years younger than her actual age because of her height (at age twelve she's only four foot two, making her a half foot shorter than Naruto when he graduated) and delicate features. It doesn't help that her teammates are three years older than her making the age disparity more jarring. It's implied and later stated that she's so short because of both genetics and early childhood malnourishment caused by the shoddy conditions of the orphanage where she was raised.
- Pony POV Series:
- Taken Up to Eleven with Princesses Celestia and Luna: they're actually older than the universe. Their physical bodies are 'only' a couple thousand. The same applies with all of their siblings.
- The Elders are even older, having created and outlived the universe multiple times. Their default avatars seem to be foals for Havoc, the Father, and Fauna Luster and an average aged mare for Entropy. Maud Pie actually is a mortal incarnation of Entropy and becomes Entropy's default avatar.
- The Chaos Six in Dark World are over a thousand years old, but frozen in the age they got the Elements of Chaos.
- Diamond Tiara and Golden Tiara in Dark World are just as old, but were made immortal by Discord when they became his minions.
- Part of Discord's curse on Dark World Derpy kept her alive for a thousand years. When she's freed, she's in her prime.
- Scootaloo and Granny Smith are both resurrected in Dark World following Discord and Nightmare Eclipse's final defeats, but chose to come back as foals despite being an adult and old mare respectively when they passed. Scootaloo to get back what Discord took from her and Granny Smith because she got a second chance at life, why not start off young again?
- In The Sage's Disciple, the protagonist Crow is 23, but has been mistaken for being as young as 14 before despite being a full grown adult. It's a point of irritation for him, but one that he can't do anything about. He's mentioned that his mother and grandfather both had the same issue well into their 40s.
- Neopolitan (on the left in the picture) from RWBY could easily pass for a teenager when she pretended to be a student, but is a victim of Vague Age in canon. How old is she in The Makings of Team CRME? Twenty-five!note For reference, her boss Roman Torchwick (to the right) is depicted as thirty years old in the fic series. In Roman's Empire where she first appears, she says she's twenty-two when Roman is twenty-seven and he has a hard time believing that she's only five years younger than him. When Mercury and Emerald (who are eighteen at the time) meet her in CRME, Mercury expresses disbelief that Neo is older than them. And before that, when Roman tells Cinder what she looks like when she needs to find her, he says Neo looks like a twelve-year-old.
"There is no way she’s actually that old! She’s probably fifteen, tops!" - Roman's narration after finding out her age.
- The Heart Trilogy: Kathryn is nineteen years old when she first meets Smaug, but after she's magically connected with the dragon, his magic keeps her from aging physically. By the time of the third part of the trilogy, she's about 140 years old.
- In the Sonic the Hedgehog fic Snow Angel, it's mentioned that Maria is almost thirteen however her illness means that she has the build of a girl several years her junior.