> if 80% of women are done with children by age 30
Is this assumption based on anything? Not saying you're wrong, after all the majority of the world's population live in poor countries where people have children younger. But at least In my social circles it'd be more accurate to say 80%+ of women start having children at age 30 (or later) then are done with it. And I know multiple women who had their first child at 40+!
No, the number is made up, I'm saying that there's a point where the advice makes sense. Whether or not that's actually the case now is then a matter of statistics.