Typos notwithstanding, there's no physiological evidence that I'm aware of that maturation must happen after 18. In fact there is much evidence that in many times and places it happened, and still happens, much earlier. So the problem isn't that college freshman are too young, but that they are too immature.
As the parent of a child accelerated significantly in school, this is a pressing issue for us. My child must be more mature than her peers if she's going to be accepted into classes with older students. Ditto if she expects to start college early. Obviously there's such a thing as too mature too fast, but at the same time there's no developmental reason not to teach those skills earlier and more rigorously. Many cultures have done so and still do.
There is actually some cognitive development that
does occur between late adolescence and adulthood. The most significant development there happens in higher order cognitive skills such as metacognition and executive function.
Now,
personally I have two minds about that subject, and neither hypothesis is actually anything like tested, so far as I am aware:
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.
Actually, that second point is one worth exploring a bit, because I have also seen evidence to suggest that students with the highest cognitive ability also tend to be the slowest to fully develop in terms of the basic neurodevelopmental arc.
