Good question.
Since I have proudly kept my virginity, I will say that the first time I could have actually lost it was when I was 16. It would have been with a girl that I was doing a biology project with. I was totally stunned when she leaned over in fron of me and let me look down her shirt. After a few seconds, she walked away and said "My family will be gone for the weekend. You can come over if you want."
Naturally, it was very tempting, as she was an attractive girl. Blond, blue eyed, nice cleavage. But I was 16, nervous and had never even kissed a girl before. I was sure it would be disastrous if I did it. Not to mention what consequences might happen. She had a bf and I was after another girl. I still think about it but I made the right decision.
See, I made you go there and then had a boring answer.