My parents were eachothers' first partners. They started dating when my mum was 16, my dad 18. I was born two years later. They're still together now almost 30 years on.
I, on the other hand, was "in love" at 21 with a guy I met on my uni course. We stayed together for 3 years, but it never really went anywhere and looking back the relationship was, if not abusive definitely not healthy (by the end I was basically just "sex on tap" for him). But I think that the reason I put up with this for so long, is that it was my first serious relationship so I had no comparison, rather than the fact I was so young.