Neurologists used to think that once adulthood was reached the human brain entered a fairly fixed state, with no more growth. Get your learning done at school boys and girls, because it’s all downhill from adulthood onwards, with an observable steady decline in the number of functioning brain cells every year.
It’s true that some kinds of learning are definitely more easily acquired by young brains, such as languages, and language-like pattern-based learning like music and mathematics. In the 1990s however greater understanding emerged, regarding neuroplasticity — the way the brain can continue to learn and evolve throughout life, provided it is stimulated in the right way. This wasn’t understood until the availability of fMRI scanners enabled researchers to see how the brain sends messages from one neuron to another, and finally identified the mechanism through which brain-injured adults were, amazingly, sometimes able to relearn functions and behaviors lost through the damaged part of the brain, by re-routing through other pathways.