I was 20 when I met my first love, 14 years later, ten years of marriage, a nine year old son and six year old daughter and we still love each other as much, in fact a hell of a lot more than we did then. We have been through hell together, more than most couples would go through in a lifetime yet we are stronger than ever. I knew after our first date that he was my soulmate and I am still sure now.
I think though that often couples who have been together from 14/15 end up feeling like brother and sister. It can be hard to break up because both have become like family members to each others families. I know three couples like this, they loved each other and through wanting to spare their families feelings stayed together and got married. All three marriages failed very early, they had grown apart as they grew up becoming more like sister and brother. My friend at 23 is now loving living the life she missed out, when trapped in a relationship she was unhappy in.
Saying that I also know couples who have been together since they were 14 and are still as much in love thirty years later. I think like any relationship, people either grow together or grow apart as has already been stated. This can happen at any stage however, not just in the teenage years.