Maybe you're just not sexually active. Not that I'm saying you should be
This sounds like a late bloomer problem. Maybe you just haven't developed on the inside yet. I mean some people are older than me and still can't get it up. But that's what Viagra's for. Anyway, you shouldn't be worrying about this too much.
If the problem continues to dictate once you are 18, then you definitely have a problem. Just wait a year and hopefully you will develop a little more on the inside down there