Due to her broken mask, Nelliel Tu Odelschwanck (Bleach) initially kept unwillingly reverting to a weaker child form whenever she exerted herself...
...but thanks to a device made by Kisuke Urahara she was able to control her age shifting.
Eon (Ben 10: Race Against Time) can hit people and objects with temporal energy blasts that causes rapid aging.
Hikari Sakurada (Castle Town Dandelion) commonly uses her power, God Hand, to age herself into a middle school student.
By speaking the magic word Shazam, William "Billy" Batson (DC Comics) can transform himself into a costumed adult with the powers of superhuman strength, speed, flight, and other abilities.
Shen Long (Dragon Ball series) is able to return the wisher's age to the requested, as both Great Demon King Piccolo and Lord Slug both wished to return to their physical prime.
The Ultimate Shen Long (Dragon Ball GT) can regress one's age to requested, as Emperor Pilaf accidentally wished for Son Goku to return to his child form, a wish so powerful that it cannot be undone by the regular Shen Long.
Having been transformed into a child by the Black Star Dragon Balls, Son Goku (Dragon Ball GT) rapidly ages back into an adult whenever he transforms into a Super Saiyan 4.
Clockwork (Danny Phantom) constantly shifts ages between an infant, a young adult, and an old man.
Pollux and Castor (God of War) can shift their age from being an old man into a younger self.
The Russian (Heroes) can alter his age at will.
Kaolla Su (Love Hina) ages from a young teenager to an adult when in the presence of a red moon.
Tsunade (Naruto) can use her mastery of the Transformation Technique to appear as a young woman, and also to alter her age to evade her gambling debts.
When Steven Universe (Steven Universe) begins to focus on his own physical aging and mortality, his gem-powers cause him to age forwards and backwards at a rapid pace depending on his mood.
Mad Mod (Teen Titans) uses his technological cane to switch ages with Robin...
...turning himself young, while Robin turns old.
Headmaster (Valkyrie Crusade) is the magic school headmaster and is a wise old sage.she uses her magic to look young.
Koenma (Yu Yu Hakusho) can freely shift between a toddler and a teenager.