Teen marriages can and do in fact last. It's not necessarily the age but the character and personality of the teens involved. Sometimes teens who marry early grow and mature at a relatively even pace and along the same paths. However, some mature differently and grow in different directions. These are the marriages that will struggle most.
They do, actually. I know of a few that have gone the distance. I think it is rare though, as so much changes from your teenage years to your later adulthood, it's just hard to keep a love or a marriage intact for that long. Many teenage marriages come about because of a child, too, and this makes it even more difficult for a young marriage to survive for long. Many people say that a young couple should get married if a pregnancy happens, that it's the "right thing to do", but I'm not sure that it is the best for all involved, especially for the baby to grow up with two parents who perhaps never loved each other.
I personally know of a fair share of "high school sweethearts" who are still married right after graduation. I graduated 10 years ago. I think I know an equal number of marriage relationships that happened either right after high school graduation or right after college graduation, that have already ended in divorce.
Like previous posts have stated, statistically speaking, high school relationships are not predicted to last. I personally think it is important for people to discover who they are and love themselves first before they can even think about loving and accepting someone else for the same reasons. I suppose if my own daughter one day wanted to marry her boyfriend from high school, I would probably encourage her to continue dating him while she persued her own life goals and dreams. If the two could stay together then it was probably meant to be.
As the previous posters noted, it all depends on the relationship. I would argue that teenage marriages are less likely to last, due to level of immaturity involved in teen relationships in general. Teens often do not have realistic expectations, thinking that every minute will be an amazing journey in the marriage. Also, teens are more likely to struggle financially, which can be an incredible burden on any relationship. Finally, I think pregnancy/children are often factors in teen marriages, which is never healthy for a relationship. However, having said that, my best friends have been together for 18 years, since she was 16 and he was 18. They did not marry as teenagers, but they have remained together in a healthy, loving relationship the entire time.
Of course some teen marriages do last, but some don't. That's true of all marriages, though. Many of those that last were from another generation, it seems to me. My parents got married young and just celebrated their 51st wedding anniversary. Many of my former students got married young, and most of them are now divorced. As with any marriage, it's the level of commitment--not the age--which determines whether or not a marriage will last.
Although teenage marriages do not tend to be as succesful as ones where the people are older, it is not like they cannot work. It is just that the people are probably less mature on average and are not as able to make good decisions about their futures.
There are a couple that are friends of mine who have been married since they were teens. In fact, they got married because she got pregnant when she was a senior and he was a junior in high school. That was 25 years ago. My wife and I weren't teens, but we were 18 when we met and 20 when we got married. We've been married 20 years now.
So it's possible for teen marriages to last...