Demographic trends and improved medical care undoubtedly have something to do with it, but it seems to me that this is an aspect of a more general phenomenon, our culture's obsession with celebrity. It used to be that a company could promote a CEO from within; now they apparently believe they're doomed unless they can recruit a "star" to guide them. With that attitude so wide-spread, it becomes harder for new people to break into the club; therefore the club's average age rises.