For me, it actually was when I 14, and was wrestling around with some friends of mine. There was one guy there who was bigger then the rest, but I could out wrestle almost anyone else, so I went for it. Of course, the difference in strength became apparent, as well as skill. (He was on wrestling team, and of course, there's no girl's wrestling where I grew up) I felt I put up a good match, but he ended up just destroying me in the end. But, despite the pain, it felt really refreshing in a strange way. So much so, that I invited him to my house later that week when my parents had gone out of town for a mini-vacation/second honeymoon, (I barely knew him, but my friends had told me he was a pretty good guy, and I trusted them) and we wrestled again. It was when we were rolling around in the living room that I realized it felt really good to be so controlled by someone so much stronger. of course, I made a habit of inviting him over whenever the oppotunity arose. After two years of this, was when I first let it go a bit further. I started searching for it wherever I could, but of course, I was about the only girl interested in wrestling of anykind. I did manage to goad a couple of my volleyball teammates into wrestling with me, convincing them it'd be a good workout as well as blowing off steam. It was then, while watching the two of them go at it, and watching one just destroy the other, that I realized that it got me, as someone said before, hot and bothered.
Edit: Forgot to add, that I enjoy both being on the giving side, and receiving end.