I really believe this as I've seen it a lot anecdotally, women who have had babies before 35 have few issues having more after 35 and even 40+, it seems the younger they start the older they can go to, like the body is practised at it. But first baby after 35 is very hard, and if they do get one after 35 subsequent conceptions and live births will be significantly harder to achieve. It's like the body doesn't know what to do or it gave up hope, thinking all that reproductive stuff is useless. I've only had cursory searches but I've yet to see any real evidence or anyone of any authority or expertise spouse this theory.