Sauron was, as many people will tell you, one of the Maiar. Here is what Tolkien writes of the origins of the Maiar in the Valaquenta:
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With the Valar came other spirits whose being also began before the World, of the same order as the Valar but of less degree. These are the Maiar, the people of the Valar, and their servants and helpers. Their number is not known to the Elves, and few have names in any of the tongues of the Children of Ilúvatar; for though it is otherwise in Aman, in Middle-earth the Maiar have seldom appeared in form visible to Elves and Men.
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As such, Sauron would be older than the earth, just like the other Valar and Maiar.