http://finalfantasy.wikia.com/wiki/Circle_of_Knowing

◾Some believe the Archons either caused or rectified the Sixth Umbral Era 1,500 years before the game.

So, with the ability to magically halt/slow aging, they could easily be 1500+ years old. Plus, if you look at all of them, they all have blonde/silver hair. Maybe that's due to age. who knows, it's just my personal theory.