I actually believe it isn’t about the over-population issue but more of an economical and career issue and that was from the research I came across several years ago on the issue. Couples are just waiting longer now to have children due to wanting to have money and career. This in turn limits the rate of births by married couples. The other issue is that at the time I was doing my research the lamestream media came out with a study that said that the older the man is and the woman is the more likely they are to have a child who would not be healthy and have some sort of disorder due to the parents age, for example the woman was over 40 and the man was over 45.
I believe that scared the millennials but since they are waiting to have children later in the marriage combined with the fear of having a child with a significant disorder once they passed a certain age, this in turn limits the number of births per couple.
I once read about a couple where the husband was over 100 and his wife; while in her nineties; had been barren for decades.