Why do some 13-15 year olds want sex?

[Maya]

I'm 13 years old and I find myself constantly wanting it.
And before someone says it's because of pressure from media or peers, it actually isn't. I haven't felt any pressure into having it. This came naturally and slowly, but I'm still confused as to why I feel it at 13.
I have a boyfriend, and I know people will say "You can't have a serious boyfriend at the age of 13!" Well, it's as serious as it can be when you're 13. On a daily basis, we often share our feelings and thoughts with each other, and we both feel that same wanting. Again, my boyfriend and I aren't the type to give in to pressure or media hype. Our wanting came naturally, and we found ourselves craving sex from each other. I think about it often, and he does too. (BTW, before anyone calls me a slut, I haven't had sex or done anything sexual yet)
Despite my urges, I made a promise to myself to wait until marriage, and to wait until I have my education and my career set. But why do I crave this at 13 years old?