FYI, ping!
I don’t think the problem is, specifically, age, short of menopause. The bigger issue is what you’re doing in all the years between puberty and 35 or 40: contraceptives, abortions, STDs, smoking.
They couldn’t possibly find a study population of men and women with healthy reproductive systems to draw meaningful conclusions about fertility “in the natural state.”
The thief?
Contraception
In my family the waiting but aging women are passed or near the point of no return..... no baby.
What has happened is that they may have decided to just not worry about it. Sister has a child (my grand child) and we’ll shower him with affection, attention and gifts is the strategy.
In Darwinian terms, by nurturing the sibling’s child they are preserving their own genes.