The Fountain of Youth was a hidden underground spring that provides eternal youth to its drinker, due to the magical water that it creates.
Hector was the guardian of the fountain, given this job by Juan Ponce de León 400 years ago. Ben as Baby Heatblast evaporated all of the fountain's water by going supernova hot in Don't Drink the Water.
According to a molecule reader, the waters of the fountain consist of a ratio of one Hydrogen atom to twelve oxygen atoms, making it HO12.
The fountain of youth provides the drinker eternal youth as long as they keep drinking its water. Thus, the effects of the water are only temporary.