Good grief! Go to your sensei and tell him / her IMMEDIATELY what you've just told the message boards. Harassment is harassment - all the worse because you are under 18 - and it is illegal.
If your instructor doesn't tell this guy to stay away from you, please stay away from him, and please also consider finding another place to train where that sort of behaviour is not tolerated.
If you feel strong enough to confront your harasser (which you must do in public), ask him if he realises that you are only 15 and tell him that you find his comments highly inappropriate.
Nobody has the right to treat you this way. Stand up for yourself and take action NOW!
Good luck, and please let us know how this situation works out for you.