I've been thinking about this thread and, the thing is, that one enters ones teens as a child and leaves them as an adult, so people change a huge amount during this time.
However, while I believe that most teenage relationships don't last, I know that some did ~ and do.
I met my husband when we were teenagers ~ 17 & 18 ~ and we are still together now that we are in our 50s ~ the same for another couple of friends.
And I know another couple, who started dating at 15, and are still together ~ in their late 50s.
My cousin thinks that it is wonderful, because we have so many shared memories. And that is absolutely true!