Girls are just generally pretty prone to reproductive problems. Even a girl in her late teens may get ovarian cysts and such.
It's not all that uncommon that you'll encounter a situation where a couple is in their late 20s and already the girl is saying, "I can't ever have kids because my reproductive system is too fucked-up." Or, she's having trouble getting pregnant and the doctor is saying, "Before we run a bunch of tests, just keep trying to get pregnant for another year and we'll see what happens." The process of trying to troubleshoot and fix whatever is keeping her from getting pregnant can be complex.
About 95 percent of 30-year-old women have only 12 percent of their original number of ovarian follicular cells, which can develop into eggs. https://www.huffpost.com/entry/age-and- ... _b_2850439
The easy solution is just to marry a young chick. The older she is, the more opportunity there has been for stuff to go wrong with her body that would render her infertile.