Rejuvenation

Rejuvenation is the ability to reverse the aging of a living being to a more youthful state, usually through magic.

In the Smurfs comic book story which was published by Marvel Comics, "The Smurf Of Youth", Smurfette looks for a fountain of youth controlled by a genie, who in actuality is Gargamel setting up a trap for the Smurfs to fall into, until Azrael shows up and ruins the plan. 

In the cartoon episode "The Fountain Of Smurf", Papa Smurf rejuvenates by some magical water from a Fountain Of Youth within a cave, which is given by the Smurfs as a gift. He becomes a Smurfling with a child's mind and playing all the time, annoying the Smurfs. The Smurfs decide to go back to the cave where they collect the magic water to let Papa Smurf return to his normal age. But the troll inside the cave gets Papa's magic book, which the reversing spell cannot be performed without it. The Smurfs soon find Gargamel and gain his help. Finally, Papa turns normal again after the reversing formula is done.

In "Smurf Van Winkle", the Smurfs played a trick on Lazy to make him believe he has slept for a few centuries while they and the village aged around him. Lazy tried to cure them by using a potion that made magical doughnuts that would reverse their aging back to being his own age, but not knowing that they were disguised as old Smurfs, they were actually de-aged to Smurflings, including Smurfette. Fortunately, Papa Smurf restores them to their proper ages by using a magical potion that he puts into Gargamel's cauldron, though in in the process it also aged the evil wizard into being an old man.

In "The Tear Of A Smurf", the witch Priscilla sought after two Smurfs who would give her the necessary ingredients for a youth-restoring formula after she had been turned into an old woman by her mother Chlorhydris. Although the two Smurfs she had captured, Jokey and Grouchy, were initially not able to give Priscilla the tears needed for her formula because of their particular natures, they eventually did so and thus the witch's youth was soon restored.

In "Gargamel's Second Childhood," Gargamel and Scruple switched ages by pulling apart a twisted cloth containing the necessary ingredients. Gargamel turns younger and Scruple turns older. The only way to reverse the spell was by recreating the process by which it first occurs. However, Papa Smurf tricked both Gargamel and Scruple by having them double the amount of the formula's mixed ingredients put into the cloth, so that when it was pulled and the formula exploded, Gargamel became a shriveled elderly man unable to go after the Smurfs and Scruple became an infant.

In "Grandpa's Fountain Of Youth", Grandpa Smurf is rejuvenated by falling into a pool of magical water, making himself like a Smurfling. This causes Sassette Smurfling lots of trouble as she is responsible to take care of this Smurfling. Later, Grandpa is restored to normal after a moment, as the magical water can probably only make people young temporarily. Also, his beard is kept although he is rejuvenated.

In both the comic books and the cartoon show, Nat, Snappy, and Slouchy became rejuvenated by accidentally entering Father Time's magical grandfather clock that ran backwards, becoming the first of the four Smurflings. Unfortunately, there was no way to bring the Smurflings back to their normal ages again except through the normal passage of time, so they make the best of their situation by getting used to having a second childhood.

In the 2011 Smurfs film, Gargamel, who was in the office building of Anjelou watching the demonstration, called Odile's "potion" a fraud and insulted her mother, which made Odile call for security. Gargamel then took the Smurf essence that was contained in his ring and used it on Odile's mother to magically restore her youth and beauty. The result was so astounding that Odile's mother had fainted, and Odile now wanted to know Gargamel's secret for restoring youth so that she could market it.