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.

Early twenties family formation bring the norm was more of a postwar thing. The guys that came back from the war really did have to grow up fast (seeing your best friends getting blown up at 18 will do that), and they essentially had zero desire to have racous twenties filled with dating around and traveling and soul searching. They'd had enough chaos already, and were all extremely eager to settle down into a peaceful family life immediately upon their return home. The age of family formation has slowly crept back upward since then, and historically, in normal peaceful times it's usually been late twenties.