I wouldnt say its extremely rare, i started losing my hair at 14. However it didnt progress much until i started propecia and bam... rapid recession on temples and its over 3 months in.
But anyways Kiddy88 my advice to you is do NOT start rogaine, propecia, etc until you are 18. I personally wouldnt bother at all with the rogaine as its very rare to grow hair that is of cosmetic value, not to mention high risks of sheds and if you shed and are a non responder ( which is probable ) all that hair wont come back ( sometimes it does but do you really want to take that chance? )
Bottom line is the treatments for balding men SUCK. All the ones that DO work really great most of the time have awful side effects so i wont bother naming any.
I think you will be fine until you are 18, but be careful once you start a treatment you must 100% commit to it or your hair will take a hit every time. Good luck little dude, try enjoy your early teens for what its worth.. im almost 20 and would love to be roughly 16 again haha.