We were both nineteen, sophomores in college, first time for both of us.
It was a mistake. Our relationship was physically progressing forward, but my emotional investment in it was moving the other direction. At the time we finally slept together, I already knew it wasn't a relationship that was headed anywhere in the long term. So while the act itself felt great physically, emotionally I felt pretty bad afterwards because if I'd been more honest w/ her we wouldn't have slept together, and both of our first times might have been emotionally more satisfying.