I'm going to be 22 when I start, so worry not. I think Gizmo's trying to say that statistics show that those who start younger leave earlier - maybe due to things like burnout, or they may have feelings of regret that they locked themselves into such a demanding profession from such an early age. It might be the case that the older you are when you start, the longer you work because you are fully commited to your choice, have properly explored alternatives and have more life experience to deal with the less glamourous sides fo the profession. It would be that they are choosing to leave early, instead of working their time until they retire.