
Originally Posted by
Dorander
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.
The Sixth Umbral Era was caused by the Elementals. Archon is a default title given to individuals who attempt to warn of and mitigate approaching calamities. They earned the name Archon for their efforts in trying to stop the fall of Dalamud.
On top of that Y'shtola states in HW that she was born in the now abandoned city of Shalayan which wasn't around 1,500 years ago and Thancred grew up in Limsa. Plus there is Matoya, Y'shtola's teacher.
Strictly speaking you don't age a great deal in appearance between the ages of 20 and 30. Personally I suspect the reality is that they just didn't have time to revamp the Scion models beyond Minfilia's while working on ARR. After all time would have been tight. Its probably why we don't see much character development or fleshing out of a lot of those characters either. You'll note that in HW we have seen a change to that. I wouldn't be surprised if all the Scions get some background development and a facelift on their models and outfits when we see them next.