By early adulthood, most cognitive changes have already occurred. However, there is still some growth going on in the frontal lobes of the cerebral cortex.
In adolescence, we tend to think in rigid terms. Even as our cognitive ability improves, we still tend to think in terms of absolute right and wrong and seek definite answers to questions and issues that might not lend themselves to such easy analysis. We also find it difficult to look at things from a point of view other than our own.
But in our late teens and early twenties, our frontal lobes develop more completely. This development gives us the ability to think in more complex terms. We can now look at problems and situations from varying perspectives, without being chained to our own narrow viewpoints. We begin to realize that not every situation has a definite right or wrong answer, and that our opinion about something is not the only opinion that matters.
Brainwaves.com puts it this way:
The frontal lobe is the most recently-evolved part of the brain and the last to develop in young adulthood. It’s dorso-lateral prefrontal circuit is the brain’s top executive.
By “top executive” they mean that this is the part of the brain that makes the most important and difficult decisions. Perhaps this physiological phenomenon explains why we become less rebellious and more accepting of our parents and the “establishment” as we grow out of adolescence. We grow to realize that nothing is as simple as we thought it was as teenagers. Of course, such a realization also requires that we make emotional and spiritual changes to accommodate our new ability to think and perceive things in a more open-minded way. Sometimes we even miss the simplicity that a one-track adolescent mind provides.