i can;t help thinking that the cost of living has something to do with it. there's an increasing number of 20 somethings who live with parents/shared housing etc who can't afford to set up home. there's alos quite a number who think that 28 is not much older than 18 & that they still have time to party their lives away. and, sex-ed throughout teenage years, is very much geared up towards putting people off having kids.
all these things mean that people are taking longer to grow up & take on real adult responsibility.
sure, it's great to have your 20s as a time to party & build a career, then think about settling down later, but that just evades the issue of having kids.
i also think that some people have managed to 'divorce' the relationship between sex & babies. they are so used to the idea of recreational sex, that they forget that primarily, sex is for the purpose of procreation.
it is a fact of nature, that if you want to be married & have kids, particularly if you want some time as a couple, then have 3 or 4 kids, you pretty much need to marry in your early 20s. sadly, much of society seems to think that it is almost taboo to marry before 30. if people know they don't want kids, then that's ok, but if they think they may want them, they need to prioritise.
after all, a career, the fantastic house, nightlife etc, can all be enjoyed in your 40s & 50s, but it's much harder to have kids then.
in 'the good old days' people had kids younger, then life began at 40.