I think the idea of people marrying before 18 makes me a little uncomfortable. There's simply no way you could have been seriously dating someone for long enough to really know them while still getting married at 16 or 17.
I mean, I am 17 right now and I'm dating, and I'm hoping to make it last... but really, I recognize that I'm not ready for marriage. I'm fine with waiting - I think that extra time will help to prove to myself and other people that I and my boyfriend are actually serious about the relationship - and plus, that seems like what we will have to do anyway. We live over a thousand miles apart, not to mention I am going to college. Meanwhile, he is still in high school. There's just no way that would work economically, physically, and etc.
That said, though, I've managed to date him for nearly a year now despite the fact that we live so far apart, so I think we are doing better than most people our age. I've already kind of decided that I will marry him if we are still together after college. I'll (hopefully) be 25 and have a PhD by then, so I see no reason why I wouldn't be able to make a decision then. 25 is more than mature enough, and I should be more than able to get a good job.
But anyway, no, I don't think it should be under 18. Maybe in times past it made sense but I just don't think it fits in our current social structure. It's difficult enough to support yourself just being a single teenager with a high school diploma, let alone a married one. The thought of scrapping college to move down and marry someone and hoping to support myself with just a high school education sounds like a really bad decision to me.
EDIT: Also read through the comments, and saw some really good points. Someone said that if a <18 couple was confident of their relationship lasting and thought they were meant to be together, they wouldn't have a problem with waiting. And I completely agree with that. As I said, I feel pretty confident in my own relationship lasting, but I'm not exactly jumping in line to get married right away. I've got time. I don't see us breaking up before I turn 18 anyway. Something really drastic would have to happen for that to occur. But that's my opinion.
I simply wouldn't consider marrying someone if I was that worried about them breaking up with me in the next couple months.