I have read that it takes about 25 years for the brain to be fully developed.
Coincidentally, humans from the Neolithic and Bronze Age had a very short life expectancy, in fact most of their life their brain wasn't fully developed.

My question is:
from an evolutionary point of view, is there a reason why humans spend so much of their life not being fully developed even long after being sexually fully developed?

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    $\begingroup$ You are asking about the phenomenon of neoteny, the delay of development well into adulthood. You will not get a better answer than this comprehensive wiki on neoteny in humans. You will find a rather impressive list of reasons and arguments throughout the entire page, especially under the Human Evolution section. $\endgroup$ – S Pr Mar 5 at 10:35

We can say that brain and our nerve system is the first system in embryo that starts to develop and as you said this system is continuing to develop until after birth. So here is a question that why our brain don't develop completely before the birth ? Evolution has gone so far as to limit the development of the brain in the human embryonic phase and to allow it to continue into the postnatal phase. This helps the infant to be born and ease the birth both for mother and the new born because if the brain had fully grown, the size of the head would have made problem in birth. Now, after birth, the brain continues to grow and develop majorly between ages 2-3 and becomes more mature after that . The ability of the brain to grow over the years gives us the ability to adapt to different environments , learnings and new issues and other capabilities That happens with the subsequent creation and pruning of dendritic spines.

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    $\begingroup$ This partially explains why we're not born with fully mature brains, but doesn't at all answer the question of why it takes so long for the brain to fully mature, long past when the rest of our organs and systems are fully functioning. $\endgroup$ – MattDMo Mar 7 at 19:03
  • $\begingroup$ At the end of the paragraph I mentioned the one point that why our brain developing takes too long $\endgroup$ – Zahra Mar 8 at 16:21
  • $\begingroup$ Being born with a fully-developed brain would still give us those same capabilities, although one could argue that language acquisition is uniquely easier during childhood (due to the characteristics of the brain during that time) than it is in adulthood. As it is, I'm not going to downvote the answer, but I'm not going to upvote it either. I think you could go a lot farther in explaining the evolutionary biology of neoteny. $\endgroup$ – MattDMo Mar 8 at 20:54

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