If I remember right, all of the former Circle members know a special magic that causes them to stop aging. They use their "deep knowledge of the world" to essentially retain their youth indefinitely (which conveniently stops SE from having to make new character models for them).
Thancred can't do that anymore, which is why he's looking closer to his actual age after his trip through the lifestream.
At its logical extreme, the only thing that is perfect is nothing. "Pure white perfection" and how the term "erase[d]" is always used to describe what becomes of a world subjected to a Flood of Light implies it's closer to this:
http://overmental.com/wp-content/upl.../oran-rief.jpg
Minus the eldritch abominations, maybe.
... until there is nothing left. Nothingness is difficult to grasp the concept of, but that's what it sounds like to me. Al'Taieu may appear to be heavenly or "perfect," but it still exists. Not so much with what's left after a Flood of Light.
