Wait until you can deal with the consequences. Ask youself, "what would I do if we had a child and she didn't want to abort?" If the answer is "I would stay and help raise it, and do a damn good job too" then you are ready. I say this not only because it is the responsible thing to do, but because if you love someone enough to stick with them even when faced with this sort of difficult decision, then you love them enough for anything.
With my first boyfriend, I waited 4 years because I met him when I was only 14. Once I had graduated high school, I felt like I could handle the consequences and I was ready, so I got my birth control and began my sexual life. But this guy I was with turned out to be a spineless twerp once out of his protective childhood bubble, so we broke up.
With my second (current) boyfriend, I had known him as a friend for several months and already knew he was rock-solid. There was no doubt in my mind whatsoever that he deserved my trust. We also fell deeply in love very quickly and in less than a month after our first "date" I felt closer to him than I had to my ex after 5 years. I saw no reason to arbitrarily postpone sex so we went right ahead.
These are just examples to show how different relationships go different ways, while the principle remains the same: love, trust, and personal responsibility are the requirements. If you've got all three, why wait?
