My understanding is that the phrase refers to the fact that athletes and others go into coaching/teaching when they can no longer do the sport (etc) because they are too old.  It was not intended to suggest that teaching is a last resort job for losers who can't do anything else.  It means when the body can no longer stand up to the rigors of professional dance, professional sports, or anything else that is a young person's game, age and experience mean they still have something of value to offer.