I was 18, it was a one night stand and I don't regret it.
I was VERY limited in my sexual experience, or experience with people of the opposite sex, because I was for some reason terrified of sex and all things sexual. It was mostly self esteem based - I figured I'd be no good, unattractive, etc, etc. I just didn't think it would be a good experience. So I just decided one night to get it over with, with someone who's opinion of me I didn't care about, and was unlikely to see again. We were safe, we did our thing, and that was that. It opened up a whole new world for me when I realized sex really wasn't a big deal. I met my fiance 6 months later, and I guarantee things would have ended up much differently if I had not done what I did - I wouldn't have allowed him to get close, and that would have been the end of it.