I also think it's the extended "adolescence" that permeates many modern western countries. I know young people who are approaching 30, barely out of their parents' house, own nothing, and still have not really engaged with being the owner of their lives. The idea of having kids is nowhere on their radar; it's just something they see as totally daunting in the amount of work they imagine it would require.

I think the time to have kids is when you are young and full of energy and still have youthful fearlessness about the future. Fertility for women starts dropping at around age 25 and certainly by 30. Then you're dealing with seeing doctors trying to get pregnant, the stress of careers, approaching middle age, and declining energy levels.