Thoughts on Mental Growth
I understand the notion that people don't grow much after the age of 20, but I'm not sure the similarity of physical and mental stature is valid. I'm more inclined to think Alasdair MacIntryre (a professor at Notre Dame University) has it right:
"Human beings, as the kind of creatures we are, need the internal goods that can only be acquired through participation in politics if we are to flourish." [Taken from Edward Clayton's, "Alasdair MacIntyre", Internet Encyclopedia of Philosophy]
In it's present state, our political infrastructure does not accommodate public participation in the political process. We have no way of knowing how our intellectual capacity will expand when we institutionalize meaningful public participation in the political process. We can be sure that a portion of the people, perhaps a large portion, will not participate productively, but some of them will blossom when their reason is consulted and they learn they can persuade others of the value of their ideas.
(Note that voting isn't "participation in the political process". On the contrary, it expresses our status as subjects of those who defined our options, because those who control the options control the outcome. Voting is an emotional experience, not an intellectual one, and we suffer badly because of it.)
So, while I agree that many people "remain stupid their whole life. They have no concept of self-improvement, personal growth, perpetual learning...", I believe a factor contributing to that circumstance is that they have no venue for intellectual growth. I also believe society will benefit when we create a political infrastructure built on the minds of the best of our peers.