A recap from a bonus disc that someone put up on youtube indicated that Eorzea has always had more aether, and the Primals are only possible BECAUSE of it.

I'd thought the Allagan did it, too. That they turned Eorzea into a giant aether prison for some reason, based on what I heard of the Binding of Bahamut.

The Allagan empire was a Third Astral civilization though. That's four ages of indeterminate length before they show up on the scene and break Bahamut on a wheel.