'Male pattern' baldness - receeding at the temples and crown in varying degrees is hereditary, and affects around 50% of the male population. Hair loss at age fifteen is more likely to be the result of a medical condition - lack of vitimins, stress, or some similar malaise, and should be checked by a doctor.
I went bald at 17 (or started going bald at 17), my dad went at 18, his dad at the same age. There can be non-genetic reasons for the early onset of baldness, but just because it happens early doesn't mean that it's anything other than male pattern baldness. Some theories say it's to do with the levels of testosterone.