Age is one of the big issues. The brain's fairly good at routing around problems when you're young and the brain's flexible; it's why young polio patients were

Age is one of the big issues. The brain's fairly good at routing around problems when you're young and the brain's flexible; it's why young polio patients were able to learn to walk again even after spending months in an iron lung. When you get older, because there's a lot more well-established connections, the brain becomes less capable of adapting to sudden loss of functionality.