DH and I were 29 and 20 respectively when we married almost 10 years ago. We had only been dating for just under a year. I don't for a minute regret marrying so young. I think if you come from a similar religious background, know that you share the same system of morals/values etc and both understand the serious commitment required to make a marriage work then you're in with a good chance of being able to go the distance. We both try to be Christlike in the way we treat others and in our actions - if both parties do this, it will increase the chance of success. We did pray about our decision, and both felt it was right.
No pre-marital sex actually meant that we didn't base our relationship on physical attraction or libidos, which can fluctuate and alter based on moods, hormones, child-induced sleep deprivation or any number of other factors. It was based on whether we were compatible in other ways.
Recently, I was thinking about the pros and cons of settling down with somebody whilst you are young and realised that the older you get, the more set in your ways and independent you become so the transition from being single to being in a long term relationship can be harder, if you are not used to compromising, putting someone else first etc. In this respect, I think it's actually quite a good idea to marry young, as long as you choose wisely and you are both committed to making it work.
I only know one couple out of all my Church-based acquaintances of a similar age to me, who divorced. The rest of them are happily married. Sure, everyone has little niggles, but they are genuinely happy in their relationships. Having come from a family background littered with divorces (my mum has married and divorced 3 times for example), I am very grateful to have married someone who shares my beliefs and I think that is what makes a huge difference in my relationship, as opposed to all the unhappy relationships within my family. Sorry for the long post 