Small misconception.
The brain continues to develop into 25-26 range, and technically doesn’t hit “maturity” until the mid 40s. This is true.
But note that “maturity” and development are spectrum terms. Your brain hits an adult level of cognition between the ages of 16 and 20, on average. The continued development takes place in various cognitive aspects but as a whole your brain is “adult” around this point.
There are not many major cognitive milestones left after that, just small refinements.
emg, so after 30 the brain falls into a state of stagnation, and then begins to degrade altogether (I’m talking about the physiological aspect and not the psyche in the first place). As for learning, the level of life experience, well, yes, probably by the age of 30-40 it is maximum, and after that a person begins to gradually lose memory and this happens faster than he has time to learn something new because nerve cells die faster than they are replaced by new ones.
But I’m talking about something else, I’m not talking about gaining maximum life experience, but about the fact that, damn, how can I explain this. Okay, I’ll try this: there is a certain structure in the genes, according to which we should look exactly like this and acquire this or that appearance, but there is also a set of sequences, so to speak, an assembly sequence, it’s like assembly in a factory, after which there is a certain resource each detail and then the shelves begin. It’s the same with the body, it goes through assembly from the embryo, to the baby, and then to the child and to the adult, and breakdowns begin. The brain also has instructions for assembly, first the ancient parts of it are assembled from an evolutionary point of view, we once fought with worms, then with fish, then with reptiles, then with rodents, then with primates, then with humanoids, and now we became humans. As the brain grows, it receives one evolutionary “layer” after another, and even in the form of a person there are different evolutionary steps and stages of brain development. So I’m talking specifically about physiological development, and it entails psychological development, yes, this is a simplified model, but it’s better than considering us as a random set of genes adapted to specific circumstances, since in this case you’ll have to consider a bunch of other variables. That’s why I’m talking about the age of 20-25 years.