According to legend, how was the fountain of youth built or created?

I don't know.

The existence of such a fountain is the entirety of the legend. The legend is unable to locate the fountain; it certainly does not go so far as to explain the origins of the fountain. How could the legend include such details without the location also being known?

A very (very, very) good story that includes a sort of "fountain of youth" is Lost Horizon. Although the classic black-and-white movie is very good, the book is tremendously better.

You might be better off with the Biblical story of The Tree of Life - created by God and located in the garden of Eden. After Adam and Eve's sin, God set angels and a flaming sword (apparently, an animated flaming sword) to guard the tree and the entrance to the garden from humans.

In Norse mythology, the "apples of Idunn" are the source of godly immortality; in Greco-Roman myth, ambrosia is the source.

- Jim, http://www.life-after-harry-potter.com/

He was. But that actually created the Fountain of Jerkassity.

The fountain of youth wasn't built or created. It was something invented by crazy #$&@'s who wanted an excuse to run around in the jungle.

They kidnap baby's and teenagers from mothers and made them drown to death in fresh clean water. Therefor creating the Fountain of Youth.