Think of it like this; if you were born in the 1950s, ‘everyone’ did the early toilet training, early eating, talking, walking, etc.
If you were born in the 70s, your parents probably followed the same rules, to some extent but with less focus.
If you were born in the 90s, as society decided that it did not want bright, active, questioning individuals, the slowing of maturity was mandated. This is linked to the sense of dependency.
In the ‘ancient’ days, twenty one was full adulthood. One was independent. In fact, no one expected an eighteen year old to be supported by mommy and daddy unless s/he was in University.
Today, finding the forty year old baby who lives with mommy and daddy and expects to be taken care of is not unusual.