I got married at 20 to my HS sweetheart; he was 21 when we got married. We got together when we were 16 and 17. We are still married 10 years later.
My family was happy when we got married because they knew him well, and loved him. We often laugh and wonder how many people thought we got married young because of pregnancy, haha. That wasn't the case for us; we just wanted to be married to each other.
We have had some rough patches, especially lately because we have literally had to make it through major shifts in going from teens, to young adults, to adults together. We are like completely different people from when we first met because of growth, but we are still fundamentally the same people. So we've had to learn how to mesh that together. It's not easy, and many do not make it through.
Also, because we got together so young and immature, we had a lot of dysfunctional habits when we argued. It takes a conscious effort to stop a behavior habit. I strongly believe that for a couple to make it when they got together young, they have to continuously make their relationship grow and mature right along with them.
Never lose respect for each other and never forget boundaries. I think a lot of couples who start out young together get too comfortable with each other in these areas. Yes, I said too comfortable. I used to think we had a strong relationship because we shared everything. I realized down the road that it's far more beautiful to be separate people who love and accept each other for who they are, and respect those differences rather than becoming one being who shares everything.