I have already seen a few young marriages fall apart, but I think it was because of them beginning for the wrong reasons (i.e. money, desperation, pregnancy, etc.). I married for love before I was even 17 1/2 and to this day, it's one of my best life decisions. I think if you're at the right place in life and are completely sure, why not? I graduated 2 years early and had been supporting myself since then. He was a couple years older and we fell in love. Now our feelings still grow every day, we can't stand to be apart (but Iraq's forced that one a little), and I've never regretted this decision. He's taken wonderful care of me, at least done the best he could, both emotionally and physically... and I do my best to reciprocate.
We did it fast, insanely fast, but it was right for us. So while some people may not be ready or may be doing it for the wrong reasons, not everyone is alike. We're not rich(complete opposite), we're not actually crazy, we are in love. And for the people that have a "we can always get divorced" attitude: I completely disagree with you. You have to have a love big enough to be willing to do anything, work through anything, and stay through anything. Both people need this attitude.
I was not going to wait. Marriage is not just a piece of paper. It is a legal contract and a promise, it's something that you cannot change if you respect the concept of it. I got married at 17 and am now saying it's not always a mistake. As long as you understand what you're promising, why not? I mean, just as long as you're truly ready.