I meet my husband at 15 in highschool, we married when he was 21 and I was 20 (right in the middle of our birthdays...) and we will celebrate our 5th in May. Aa a general statement I would not recomend marriage at an early age; I'm a freak of nature/nurture who was very mature for my age and extremly independant. (How many teenagers do you know who refused free cell phone/credit card/etc from their parents because they prefer to shoulder the responsibitiy themselves?) To echo an earlier post, it is helpful to have lived independant lives before marriage and not to go srtaight from living with parents to living with spouse. My sister married the month after I did at 19, and I don't really expect her to make it (and she has kids; sad). Maturity and realism has a lot to do with how a marriage will last; so does ambition (on both sides), how people comunicate/fight, etc. Okay, there's a lot of factors involved... So I'll just stick to why it works for me.
Again, I lived independantly before marriage; he was in school and I worked. I have been the main income producer up until the last year and a half; now I am in school. We moved across the country for me to attend school, quite a sacrifice for a man to make, but he did it gladly. Our goals in life are similar, our career goals in the same field (me: theatrical/film MUA, he; theatrical/film electrician). We communicate very effectively and fight fair. We don't try to make everything half and half; there are just some things i do better than him and vice versa. And we laugh. ALOT. I could never have married a boring man, and even as I write this, he is trying to get me to laugh. ; )
So besides maturity, I think equality helps. In my experience, people not on the same page don't last. I also recommend counseling of some kind before marriage, and visit some model homes! You may be surprised what your other half thinks is important in a home, and by extenson family, and how it meets up with your expectations. It's important to have conversations about those types of things (the model home isn't required, just fun)
BTW, the most annoying comment i get when people find out I am married, even now at 25 (i guess I look younger) is, "do you have kids?". No, we have cats.