That's great for the environment and very good for the work and housing prospects of the relative handful of kids that are being born (since fewer workers mean rising pay and fewer households mean cheaper real estate). But it's bad for retirees who will have their benefits slashed when there are too few workers to support them.
Let's begin with some charts from today's Wall Street Journal. The first shows women waiting longer to have kids, with the average age for first birth rising from 21 in 1968 to 26 today:
This is due to a few (admittedly somewhat contradictory) things. First, college-educated women tend to make more money, which raises the opportunity cost of starting a family. Second, soaring student debt for those same college graduates makes work more necessary and the added expense of kids more terrifying.