> It's much more a matter of attitudes about having children.

That is the story right there. We as a society spent decades upon decades demonizing having children at a young-ish age. "Your career is more important", they said. We got shows like "16 and Pregnant" to dissuade viewers from having children. People have become genuinely afraid of having kids.

Not until you are in your 30s does the social messaging shift from "only failures have children" to "why haven't you had a child yet?" That change in social pressure often compels one to start to change their mind, but at that point one becomes biologically limited in how many children they can reasonably birth.