There is actually some cognitive development that does occur between late adolescence and adulthood. ...
1. that executive function largely ONLY develops under 'load' and that this is why we are now effectively seeing cognitive development stretching into the 20's while physical development is going in the other direction, and
2. that UNTIL cognitive development is more or less complete, most human beings are not really well-prepared to learn at the highest level that their cognitive ability/potential would suggest.
I stand corrected then. However considering the age, historically, where people were considered functioning adults (able to marry, have children, run businesses, have careers, die in war) I'd agree strongly with your first point. Without pressure to mature, maturation does not occur. We probably all know of people who never had to mature into their 30s and beyond.
For our DD, the feedback from adults is mixed and somewhat contradictory. When she's in their class, they LOVE that she's mature. When she's not, they're equally happy to see her act her age (since it makes her appear more 'normal' and well-adjusted). She seems to be handling it pretty well so far and we sometimes put up with some additional immaturity outside of school because we know she needs a break.
Now executive function skills are a different matter. I'd say she's about 'at age' for these skills, which means we are scaffolding and pushing for her to ramp those up. Because she is going to need them.
