I think a woman's biological clock is pretty good at telling her when it's too late - the menstrual cycle stops, etc, but I know that there are heightened risks with reproducing towards the end of your reproductive span, and that has to be thought of.
You could potentially get very fit people who are 60 and live to be 104 having children, equally you could have unfit people with ridiculous lifestyles who have children at 20 but would be lucky to live to 50. You can't ever say that there's a cut off that everybody should abide by.
I would say that women need to think more about their age if they want children, though. It is perfectly possible to have a child and a career, and I think it's slightly selfish if you put the health of your child at risk by leaving it late, knowing that you're doing so.
But, as I said, it has to be a "case by case" judgement.