If the cut off age were 40 or 50 I could buy the infection, or healing theories. Given that the cut off is 30 my argument is that what really drops off fast between 20 and 30 is athletic supremacy as we see in today’s such biologic experiments. That is where you see a steep drop off starting to happen around age 30.
The “clock” still exists in humans. Female fertility ends in the forties.
And since we are “designed” around and for procreation, nature has determined that our “usefulness” and ability to support offspring ends at around that age. It takes a human child approx 15 years to mature. Maybe less.
Of course, nature is cruel. So, even if the “clock” exists, environmental factors will control longevity.
Environmental factors will affect fertility such as disease, nutrition, weight. A woman will lose her ability to have children if she is malnourished, etc. Maybe this is more of an issue when a woman is older thus dropping the age from mid-forties to thirties.
I remember reading that women super-athletes with very low fat can lose their menstrual cycle.