I got married at 18, which is far too young for a lot of people. In our situation though, my hubby already had a stable job with the military so financial stability was not an issue.
It depends a lot on maturity. Many 18-21 (or so) year olds are far too immature to get married...
On another note, I think that it is best to have kids earlier (before 35). Genetically it is healthier and plus you'll have a better chance of being around when your child grows up, gets married, and has kids of their own.