Good times make for soft people. Automation brought so much to so many for so little, we don't need to work so hard.
So we retire earlier, work fewer days of the year, shorter hours; letting our kids remain unproductive longer is just another consequence of that.
(It being wealth, not laws, which ended widespread child labour)
When the harsh realities of life aren't so harsh any more, responsibilities can be shirked a little longer, and then a little longer...
Suddenly you have guys in their 30's still living with their parents; no savings, no prospects, no pressing need to change anything, because food is cheaper than it's ever been and internet porn keeps the loneliness at bay.
Did you know that 200 years ago 95% of people were farmers; while today 95% of people aren't farmers?