The Great Tree of Possibilities
I was pondering the implication of the name "The Great Tree of Possibilities" again as I read my mother's Myst fanfiction Visitation.
You see, your average tree has a trunk from which all its branches spread. That trunk is essentially the visible starting point. Thus, the simile implies that all possibilites spring from one fact or one deed. What if that fact is based in more facts or that deed was caused by millions of others before it? When you reach the last of Tree's roots, what do you see? Those little tubers are in turn are in the soil. I think that soil is ultimately reality, truth.
I believe that even possibility must operate in firm fact.
As for how Ages come to be, I think that they probably come from the infinite mind of the Maker. Writers merely decribe places that either exist as possiblities or as fully functional worlds.