"Not until you're 30!"
I've decided fairly recently (past year or two) that I do want kids at some point (as opposed to not knowing) but not until I'm established in my career. However, I'm not sure I could say why. So I definitely wouldn't criticize sounding for not wanting kids.
Don't take this the wrong way, but don't wait. I know it's in vogue to have a career and then settle down. But to anyone who thinks they want children, I advise, don't wait. (Well, be in a committed relationship with someone who also wants children.) But once you are out of school, soon is the time to have children. You can always want to accomplish one more thing before you'll be ready to have kids. Do it too often and you'll be in your late 30s and wondering if you should have kids.
But the real reason to so sooner rather than later is that in your 20s you still have an abundance of energy and a child can suck that dry in an instant. Young people are designed for the rigors of child rearing. It will also be easier for you to identify with the trends that will fascinate your child when you are closer to their age.
And for those who think they won't want kids, that's fine. The last thing a child needs is a parent who regrets the decision to have kids, right?
Finally, I'd like to address some of the fears in this thread. Fear of the unknown is normal. Overcoming fear is also normal. Do it. There is no reason to fear "I might screw up some child's life." Odds are the child will mess with your head just as badly as you mess with its.

I find it painfully ironic that the people who generally don't want kids are the smarter people who over think everything. I'm not saying having kids should be a thought-free endeavor. But analysis paralysis tends to be the geek way to avoid parenting.
My son can be a royal pain in the ass, and he's only seven. But I love him like I've never loved before. And you can't place a value of that kind of love.
YMMV, VWPBL, and several other disclaimers.