That's not a mature hairline I'm afraid to say, as much as you didn't want to hear this, I believe that is in fact aggressive male pattern baldness , your hairline is very receded for a 15 year old and even from that bad photo I can see more thinning at the temple and frontal area.
Nizoral and Minoxidil (Regaine, Rogaine, Kirkland) is something you should consider using providing you have the cash for Rogaine/Regaine, if not, Kirkland minoxidil is very cheap and just as good I hear.
DO NOT TOUCH PROPECIA AS A 15 YEAR OLD.
Sorry for the bad news, but IT IS MANAGEABLE in this day and age, if you act fast. I'd say really fast in your current state.