I think I had my first "serious" girlfriend when I was 13, but in retrospect everything was serious and... nothing sexual happened that you wouldn't see in a PG movie (not even PG-13). We did have a heavy make-out session at a movie theater during Star Trek:The Motion Picture ... it really helped the movie.
My parents' attitude was to be supportive, and to make sure that any of their children's relationships would be built on respect. That, I think, was a very good lesson.
I also remember them buying us a copy of this book:

I haven't read it since the late '70s, so I don't know how it would play now, but it was a great resource for me during my adolescence.