If stability arrives only in your mid to late 30s, leaving only a handful of fertile years (assuming no prior medical problems picked-up in younger life) then it stands to reason that a women will be far less successful at producing a large family than one in say, her early 20s. For men, I would agree with you. But then the calculation is rather different.
It's a pretty delicate balancing act, and one that's getting harder because of the increasing complexity of modern society.
The issue is that differentiated careers - ones where you are hard to replace, your company will pay you more than commodity wages, and they'll give you time off for a sick kid because replacing you will cost more than just letting you take a day off when necessary - require a long period of time to become differentiated and hard to replace. That could be spending your 20s in med school and residency, or in law school and being an associate, or bouncing between lower-level engineering jobs before you get the experience needed to tackle big projects. But in most fields, it does take up most of your 20s, which means you really only get stability, trust, and the ability to downshift for family life in your 30s.
Ideally, on a societal level, you should be able to raise a family on one of those "commodity" wages. But that's not happening, largely because many of the necessities of life - housing, health care, and education - have become differentiated professions where owners and practitioners can get premium wages. We need to allow shantytowns and village doctors again before we can have large amounts of people having kids in their 20s.
