While Gaeta is one of the few characters we have not seen the inner thoughts of, and thus he could be an aware Cylon, the other 4 were sleepers so the 5th could be too.

However, I don’t see what you’re saying about age. Nobody appears 4,000 years old, so that tells us nothing. The F5 could plant sleepers where and when they wanted them, over the whole history of Kobol and the colonies.