In "The Survivors," we're introduced to an enigmatic and powerful being known as a Douwd. In human form, he goes by the name Kevin Uxbridge. He states that he's lived in the system for thousands of years, but is it possible to know his overall age? Perhaps from a source outside of the episode itself?
Kevin is the only Dowud we know of, and, to the best of my knowledge, never appears in any other episode, or in any lesser-canon work. We have nothing to go on but his word.