I was 20 (an old maid by the standards here, apparently), we were friends, and while we had been flirting back and forth for a year, the actual sex part instantly destroyed the friendship. (Not because it was bad or anything, just because it was weird.) So yeah, I regret it. All these years later I still think about him and am sorry for not waiting. He might or might not have turned out to be "the one," but I miss the friendship. I did it partly because I liked him a lot (more important, he liked me back -- I'm not a guy magnet, now or then) and partly because I felt like it was high time to get it over with. Not a good plan, honestly.

Whatever you decide, be careful.