Isn't menopause the clock?

The biological clock exists, and it is real. But it ends at a time where no reasonable person should seriously consider having a first child.

For most women menopause starts around 45 and the last period happens around 49-55. That's the hard limit.

Between 30-45 having kids is most often possible, though it's getting more difficult and the chance for things like trisomy 21 is increasing exponentially with increasing age.