Peter Davison was only about 30 when he portrayed the Doctor.. I might be wrong, but I think he was the only exception to the age thing though?
The future of Doctor Who is mysterious anyway, both because of the getting younger thing and because of the rule of only 13 regenerations. They are both rules they can easily break with the explanation that Gallifrey is gone and the Doctor can easily cheat the rules now, but what with Matt Smith being so young, I can't really imagine them getting an actor even younger for the 12th. We already have the Sarah Jane Adventures aimed at kids.. :/