I just watched the documentary linked below, about falling birth-rates, released on Youtube yesterday, by the guy who did the research.
(The fact in the subject is from an interview, the documentary itself only give the statistic for Japan, where the equivalent age is 26.)
Some factoids for those who won't watch the video (some are from the documentary, some are from two interviews with the maker that I've also watched.)
Birth-rates are below the level needed to keep population stable everywhere except sub-Saharan Africa. (It looks like only a matter of time until it's true there as well.)
That the invention of the pill is causing this is disproved by the fact that rates fell suddenly in Japan 20 years before the pill became legal there. They fell at the same time as birth-rates in multiple other countries, so it's not that Japan has a different cause.
Women who do become mothers are not having fewer children than before, the issue is that suddenly a large chunk of women are having no children at all. In other words, the problem is not smaller families, the problem is fewer families. (If I remember rightly, Japan went from 1 in 30 women childless to 1 in 5, in the space of three years. It's now 1 in 3.)
I think I caught a statistic somewhere that 40% of US women are now destined to be childless. (Presumably that is among those becoming adult now. But I might be wrong about this statistic, may have misheard/misunderstood.)
One reason childlessness is a problem is that 4 out of 5 women who never have children are biologically fertile and would have liked to have had children, but just never made it happen. Obviously there will also be economic issues, if each 20-year-old entering the job market has to generate enough economic output to support multiple 70-year-olds.
Even though birth-rates are falling. generations already born before births peaked will caused older age brackets to have increasing numbers of people, so for a few decades, overall population will still increase despite births decreasing.
The birth-rate of a population can be 90% predicted by the average age at which a women has her first child. The exact figure has not yet been researched, but it appears to be the case that population will inevitably decline if women who want children do not have their first child by their mid-twenties.
Immigration will not be able to solve the economic problems caused by falling population. There will be nowhere with a people surplus for them to come from. (There was a jokey interview claim that India already has ghost villages, they need immigrants!)
The cause of the decline seems to be a failure of couples to get together in time to have children. The data shows a big drop in birth-rates every time there is a major economic crisis. In response to the crisis, people postpone having children, but once society has shifted to aiming to have children at an older age, it never shifts back to having them at the original age.
A metaphor that explains why couple-formation is down. Imagine you live in a village with a dance-hall that is open for three hours on a Saturday evening. Every young person is there for the whole three hours, and gets to see every other person they could potentially marry had have children with. Now imagine the opening hours are changed to six hours, but most people still only have the energy to go for three hours. Some people leave before the person they should have met and married arrives. Some people are half-way through getting to know one person when another person enters and catches there eye, one courtship is interrupted by a new possibility. Perhaps this disruption kills one potential relationship. If the time-period during which most men and women think they need to mate has changed from maybe as little as five years to as long as 20 years, the likelihood that any potential pair will be on the same page at the same time goes down.