As JMG says, this is nothing new. Exactly the same thing was going on in Rome during the decline, as well as Egypt, China at certain stages, and as you mentioned the mesoamerican culture. The Aztecs were the final ‘Roman’ version of a far older culture that started in Yucatan, and they had all the same issues we have now in the west.

What you have to realise is it really is just late stage urban phenomenon. Go to small town areas, or groups like the Amish, and people are still having kids and continuing the generations. It’s just that so many of us live in cities now and that is the focus, but for example where I live in a rural farming area the last few years have seen a massive baby boom, where maternity wards were having to turn people away because they were full. I see it amongst my friends too, those who live out here usually have at least one or two kids, while many of my city friends are childless.