Well being 16 and having started having sex with my girlfriend about 8 months ago, I can tell you that the only time I can't cum is when 1)I'm using a condom and 2) I masturbated 3 times the day before. And i don't mind some times, since she seems to really enjoy it ("scratchin on my back, oh here she cums", like that Eezy-E song).
In fact, sometimes, she would become dry after too much fucking and I hadn't burst yet. But then she'd just suck me off or something.
My biggest problem is that then sex isn't THAT enjoyable for me since my priority is her.