I suggest talking to your doctor regarding the issue. Developing stuttering at 15 out of the blue is a bit strange. Stuttering from brain damage has occurred before. It could also be due to stress. Stress can cause stuttering too. If you've hit your head at all before the stuttering started, then you need to go to the doctor. Otherwise, it could be due to stress. The brain developing could be at fault too, although this is unlikely due to your age.