Some of that one might not be just age but how certain stages in women's lives are more likely to make them experience sexism and sex discrimination. Entering the labor force (from, say, college) is one such stage as colleges in the West are not openly sexist today, so many have little experience, outside sexual harassment and violence, of the other ways sexism crops up. Then having a child will intensify all that, make labor market discrimination more obvious and often results in women doing all the household chores and ending up earning less at work for all sorts of related reasons.

Some types of sexism also increase as women age, of course. Hiring discrimination rises with age etc.