There have been a few posts on the DIS boards that indicate new policy is to charge double-price for people over 9 eating from the kids menu at a table service restaurant (not buffet/character meal). Some say that it is for a slightly larger serving and some say that the serving is the standard kids menu size.
I haven't been there lately and don't know official policy. When we were there last (May 2004), my 13 year old was allowed to order from the kids menus in a couple of places for the same price as my younger two (then the dining age was 12 and under). I told the waiter/waitress his age and that he would prefer the kids menu, and they were fine with that.
Beth