So there's no need for a woman to have children at all, but saying "do not have kids earlier than 30 isn't great advice. There's a measured correlation between maternal age and the incidence of Down's Syndrome and other trisomies - the longer a woman waits to have children, the higher the odds there will be a problem. Which is not to say that all women should have children as young as possible - it's just that there's no one blanket piece of advice that's universally good.