Women's fertility window is much smaller than commonly believed.
By the age of 31, ~10% of women are sterile and ~50% are subfertile (i.e. they have difficulty conceiving; decreased egg quality; increased risk of birth defects, pregnancy complications, hereditary disorders, etc.).
By the age of 40, ~40% of women are sterile and ~100% are subfertile.
For 10% of women, even 25 may be too old to have a healthy child.
Sources:
https://www.researchgate.net/publication/11275869_The_variability_of_female_reproductive_aging
https://web.archive.org/web/20160419232824/http://www.oeaw.ac.at/vid/download/pce/dec02/pm/PCB05-Leridon.pdf
@Bernard
Yep. That's why historically the average marriage age for women were below 20, so they could start having children during their prime fertility window.
Plus, once a woman has her first kid, her fertility doesn't degrade as fast and they can have a much larger family. It's a win-win.
@Zeb
The results of encouraging women to go to college and pursue jobs or whatever before they get married and have kids is clear.
