A fandom example: for a while, it was vaguely popular among Harry PotterFan 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 withSnape. 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.
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 Yukide-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.
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.
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 actually 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.
Happens with Loki in this fic, where he decides to use his new appearance to hide from the alien army he pissed off previously.
