Sauron is a one of the Ainur, which are comparable to gods or
angels. The Ainur exist in two groups: the Valar and the Maiar. The
Maiar are lesser beings than the Valar and serve them.
Sauron is a Maiar, and so he has existed since the very
beginning of Ea (the world). To make a guess based on the
chronology of events in Tolkien's world, Sauron is at least ten
thousand years old.