My son received  his  2nd or 3rd degree black belt in TKD.  It was a great confidence builder and he was in great physical shape in his early twenties.   Marriage, 2 kids, work, and he quit the tae kwon do.   Now in his 40's he has had both hips replaced.    Doctors told him martial arts are great as long as you keep it up, but if you quit your knees and hips deteriorate.   Due to the stress on the joints.