I suspect it's a cultural thing as well, with most (all?) wealthy cultures veering towards individualism and working. Whereas with previous generations, the grandparents and environment would be more involved in raising children and educating the new parents.

But I also feel like people grew up or had to grow up earlier back when. My parents were married, bought a house and had kids on the way by their mid 20's, when I was that age I had just about finished my education and started my first fulltime job, it'd take another decade to buy a house. Buying a house / getting a mortgage is a major commitment, and I think you'd get a big boost of adulthood / personal development if you do that in your mid 20's.