I was 15, and it was with a great friend that I trusted very much. We were not in a relationship, and never were afterwards either. I know that's kind of unorthodox for some, but it was a decision that fit who I am and the type of personality I have. It was a great experience; I have no regrets whatsoever, and would do it the same way over again if I went back in time.
My approach of being able to separate "love" emotions and physical connection is not one for everyone, but *shrugs*. Don't get me wrong - having sex with someone you're in love with is a totally different experience, and the ONLY worthy experience for some, it just wasn't always absolutely necessary for me, in all situations.
As for age . . . I agree that now, at 24, 15 seems really young, but I felt at the time, and still do now, that that's when I was ready. Like I said; I have no regrets in any aspect.
I'm not advocating my approach for anyone who's thinking of having sex for the first time - just recounting my experience.
But no matter who you are, how old you are, or under what circumstances you decide to have sex, BE CAREFUL and be safe.