Young children have the most adaptable and flexible brains, and can usually learn a lot, quickly, but tend to have a short attention span, which limits how much of any one thing we can learn at a time. At that age we also don't really want to be forced to sit and study, so that also limits how much we learn when young. Once we hit puberty, we still have a great capacity for learning, but the hormone imbalances of this age range interfere with the learning process, and we still can't stand to sit and study. We are also struggling to master social skills and dealing with sometimes ruthless peer pressure. All of this also interferes with the learning process, sometimes well into the twenties. Once we are adults, our minds may not be quite as good at learning as a child's, but we have some life experience to couple with any new learning experiences. Also, as adults, when we study something, we do so either from a sincere personal interest in the topic, or from a chance to profit by the learning, such as a better job/career. Both a sincere interest, and a chance to gain from it, means that adults are more motivated when we study up on something. I believe that this far outweighs the fact that our minds are not as good at learning as a child's. All the capacity for learning in the world is useless without motivation behind it. The only real motivation to learn that kids have, is that they will be punished if they don't keep up with their studies. That's not the same as wanting to do it.
I was a lousy student, and will never be a brilliant genius, but as an adult, I do enjoy learning things of interest to me, and will continue to do so, whenever I want. Besides, there is no law against continuing to learn once you are an adult. There are many elderly people who have gone to college to get degrees late in life. Bravo to them! And to all who continue to learn as adults!
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If a man does not keep pace with his companions, perhaps it is because he hears a different drummer.
Let him step to the music which he hears, however measured, or far away.--Henry David Thoreau