I don't want kids for all the selfish reasons. I don't want to raise them, feed them, take care of them, I don't want my time infringed upon with them, etc. etc. As recently as this week someone asked me how many kids I have instead of if I have any kids. Then she asked me how old I was and gave me a little look. Cripes, I'm still in my prime breeding zone! The expectation that you should have kids is annoying.
I've seen some less than ideal parenting situations turn out OK. I know girls that I went to high school with get pregnant at 14, drop out of school, come back get their GED, go to college, get a job and are raising one or two kids by themselves. Of course the initial situation wasn't so smart, but they turn out to be good parents. Not that having children should be used as a maturity miracle grow, but having a kid probably was the best thing to happen to them. It literally slowed them down and made them grow up. There's no amount of planning you can do to prepare you for having a child. This should be obvious by the volume of parenting books out there. People don't need instructions on how to boil water.
Deciding who should and shouldn't have kids is sliding into dangerous eugenics-like territory. If there were a set of parameters that people must fall within before having kids, most kids wouldn't get born. I don't even know if comes down to maturity level because people change a lot (usually for the better) after having children. Duh, hasn't anyone seen "Knocked Up"?