The Fountain of Youth can be very beneficial to all that find it, not only the civilization that manages to include it within its borders. When a land-based military unit moves into a hex adjacent to the Fountain, it gains a special promotion that allows it to heal at twice the normal rate for the rest of the game!
The Happiness bonus only requires that the Fountain of Youth be in a civilization's borders, since the Fountain itself cannot normally be worked unless the Natural Heritage Sites resolution is in effect.
Try to make use of the Fountain of Youth's ability as much as possible, sending as many of your units as you can there. If it gets incorporated into another player's territory, try to get an Open Borders agreement with him or her so you can continue to benefit from its healing power! This natural wonder is also notable for being the least likely to spawn during map generation.
One of the earliest accounts of this legendary spring comes from Herodotus, who describes it as a special "Water of Life" giving any who drinks of it exceptionally long life. Similar stories have also been told among the indigenous people of the Caribbean and South America, which brought the Fountain of Youth to prominence during Spain’s era of exploration. According to one popular story, in 1513 the Spanish conquistador Juan Ponce de León heard tales of the fountain during his travels in Puerto Rico, and set sail to search for eternal life. In doing so, he discovered Florida (and became one of the first Europeans to set foot on North American soil). The Fountain of Youth continues to captivate audiences today - movies, books, comics, and games have all featured the fabled spring, and man’s eternal quest to obtain it.