I don't wish I'd started earlier, because I would've been a crappy mother in my twenties. I'm much more sure of myself now, in my thirties, and it's perfect for me.
What I've noticed is that younger mothers who have been raised in loving, attached families, are ready, much earlier, to have children themselves. They are more grounded and more mature than I ever was, at the same age.
Coming from a brutally abusive background, I needed ten years to come to terms with my own childhood, before I could provide anyone else with a childhood. The young mothers I see who are really terrible mothers mostly come from the same background as me, only they didn't give themselves time to work through their issues. I work as a volunteer child advocate at a battered women's shelter, so I come in contact with very young mothers quite a bit.
Through my LLL group, I've met lots of twenty-something moms who are just terrific mothers, but they almost all come from loving, stable families.
Maybe it's an overgeneralization, but I think the really good young moms have loving families of their own, while the older moms have come from slightly more troubled backgrounds, and they needed time to sort themselves out.
I guess it means that we should be prepared to have our own children have children young. That's cool. We'll still get to be grandparents!