I turned 24 three weeks before I got married, and my husband turned 23 a few days after our wedding. I don't feel like we're too young to be married. Actually, I think we're probably pretty average.
I'm glad we're married at the ages we are now. We want to have kids by the time we're 27-28, which gives us a good 4 or 5 years to be young and crazy, travel, finish school, be somewhat selfish with our time and money... life is good as young newlyweds.
I don't think people should get married before 21, certainly. There is a lot of growing up that happens between 18 and 21, IMO. If I'd married the boyfriend I had at 18, we would have been divorced long ago...