In Tangled, originally it was the magic flower that Mother Gothel used to keep her young. But after the Queen drank the flower and gave birth to Rapunzel, Gothel got her Fountain Of Youth from Rapunzel's
The Sequel Hook at the end of Pirates of the Caribbean: 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... •
the Fountain in On Stranger Tides is played with. It doesn't make you any younger, but instead adds years to your life. However you need someone to take the years from...
•The MacGuffin weapon of the Big Bad in Sky High did exactly this.
The titular character in The Curious Case of Benjamin Button has a similar problem: he ages backwards. He is born as an old man, and dies of old age as an infant
The "Little Muriel" episode of Courage the Cowardly Dog has Muriel being turned into a mischievous little girl by a freak tornado. It then turns out the only way to undo the effect is to throw her into a tornado spinning in the opposite direction of the original one - which entails flying to the southern hemisphere.
In Adventure Time, the (apparently) 18-year-old Princess Bubblegum suffers an accident in which she loses some of her body mass. Being a sentient wad of bubblegum in a generally princess-like shape, she is saved by being reconstructed in the shape of an apparently 13-year-old princess. Since The Mind Is The Plaything Of The Body, this also causes her personality to take on the traits of a 13-year-old. The situation is, of course, temporary
One episode of Lilo & Stitch: The Series involved an experiment known as the "Babyfier" that turned its victims into infants.