All assumptions though. There's plenty of people who claim that Sephiroth is in his mid-to-late thirties, or even in his fifties. No one can be certain, because there hasn't been an official age announced for him as it stands.
I'd like to assume that he is atleast in his thirties, and as old as Angeal or something. I'd like to think that Zack is the young one in Soldier.