I had them when I was 12 and probably when I was 10. In my case, the cause was having fibromyalgia and being forced to exercise. I had the common trigger points which are like knotted up hard spots. These areas would bulk up after exercise, and it was too much too fast, so I got stretch marks in those areas. I also had improper nutrition and hydration, but I am sure nutrition and hydration would not have prevented them in my case. I tried some creams on mine which didn't help much. By the time I tried creams, I probably had them at least 6 months.