I knew I wanted children from a very young age. I was always very maternal, doting on my younger siblings and being the neighborhood babysitter. My older sister on the other hand, always said growing up that she would never have children. She changed her mind in her 20's, and now has two boys and is very happy.
I do however have an aunt and uncle in their 50's who made the decision that they didn't want children. They are very happy with this decision, as the two of them are content to have eachother.
Personally, I think it depends on the person. If you really feel you don't want children, then you probably aren't ready. But if you are just a little unsure, then you'd probably be delighted if you had a child. Whether you decide to have kids or not is completely up to you, and a decision that only you should make ( and of course your signifigant other ).