It also isn’t rare for 17-y-o girls to run into serious bone density problems when they haven’t had regular periods up to that age. Next step, osteoporosis - a problem for life, not just for the track. It has to do with level of bodyfat. If a female isn’t carrying enough, the body will sometimes switch off the fertility machine… so although many will never run as fast again following the onset (delayed or whatever) of menstruation and sometimes also bodyweight (read fat) we should really be thankful that they’ll get onto a more healthy life track. I’m sure we would all agree on that.