My bet is that this all stems from higher housing prices / cost of living.

Previously you moved out at 18 or so and moved into a dorm/apartment with folks you age and this dynamic introduced a space where you could experiment with "vices" (alcohol, drugs, sex, parties, etc).

Now with cost of living being so insane, kids are instead choosing to live with parents and thus never get a space to experiment like this. And by the time they do have enough money to move out on their own, the "experimentation" phase is largely over.