Ok, here's my theory, and it's a doozy. Olver is really Gaidal Cain. Mat's been playing the Foxes and Snakes game with him (which, if you remember, was patterned after the old agreements and was a lesson on dealing with the people on the other side of the doors). So by this time Olver must be getting pretty good at this game. So he goes through the doorway, outwits the fox-people, and rescues Moiraine. But in the process he is aged considerably. Ta-daa! He is now old enough to fight. Oh yeah, he is also given the knowledge of how to fight with two swords. Now that I think about it, maybe it's not that far-fetched.