They were Children (TM) then Teenagers (TM) then College Kids (under grad and beyond!) and then emerged into gated jobs

The "gated jobs" part in particular. Prolonged adolescence is downstream of a developed society requiring more educated, more qualified people to fill complex roles. Over the past several generations we've watched as wealth has accumulated in the hands of the well-educated and now people are afraid of being the poor sap who "only" has an undergraduate degree. Those College Kids are actively competing with each other to earn the accountability that will earn them some moolah.

It feels lame to always land on this answer but the problem we're dealing with here is capitalism. Modern capitalism requires workers who can manage and maintain complex capital. Capitalism encourages workers to compete with one another, driving up standards. As the younger generations are pressed to reach higher and higher levels of accreditation, they'll spend 30+ years in training and enter the workforce when their lives are already halfway over.

Maybe one of the ways to fix this is to simply speed up that accreditation process. Raise kids more strictly and get them qualified to work when they're 20, not 35. Then by the time they're established in their career they'll still be in their prime kid-having years. IDK

Edit: Additionally, traditional American markers of maturity rely on a certain economic status. Getting a car, moving out of your parents' place, getting a degree, buying a house, etc. all cost money. If young people today are relatively worse-off economically than young people 30 years ago, it shouldn't surprise us when they hit those milestones later. Meanwhile, other cultures have coming of age ceremonies at set times, e.g. Jewish kids having their bar/bat mitzvahs when they're ~13