William "Billy" Batson (DC Comics); via speaking the magic word "Shazam"
Eon (Ben 10) can hit people and objects with temporal energy blasts that cause rapid aging.
Shen Long (Dragon Ball) 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) 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, 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.
Tsunade (Naruto) uses the Transformation Technique to appear as a young woman, and also to alter her age to evade her gambling debts.
Koenma (Yu Yu Hakusho) can freely shift between a toddler and a teenager.
Pollux and Castor (God of War) can shift their age from being an old man into a younger self.
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.
Headmaster (Valkyrie Crusade) is the magic school headmaster and is a wise old sage.she uses her magic to look young.
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.