The Fountain Of Youth is a legendary spring of water that is supposed to bring rejuvenation to whoever drinks of it.
In mainstream Smurfs media, the Smurfs have encountered a fountain of youth in two separate locations. In "The Fountain Of Smurf", there was one in the Smurf Forest which de-aged Papa Smurf into a young Smurfling. That one was protected by a guardian troll that took one of Papa Smurf's books in exchange for a sample of water from the fountain, and who was later de-aged along with Gargamel when he chased after the evil wizard. In "Grandpa's Fountain Of Youth", there was one in South America which partially de-aged Grandpa Smurf into a young Smurfling, though he still retains his beard.
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.