I'm talking about biological ideal, rather than current societal norms. And it's not out the window everywhere. My niece got married shortly out of college a few years back.
Before about 24, most people are not emotionally mature enough to be a parent. Fertility starts going down after 30. That leaves a fairly narrow window for women who are serious about having kids.
Working backward from the maturity/fertility window then determines the ideal time window to get married is 20-29, preferably being early on the interval
Was her husband the same age?