It is not the size of the brain that makes one person brilliant and another backward, for most adults have the same size brain about three pounds. Rather, it is the way the brain develops.
Your brain stores information from your past experiences, helping you to remember, learn, and think.
But people differ in how much they remember, how quickly they learn, and how well they think. Much of this depends on the capacity for mental growth you inherit from your parents and on your environment, or surroundings.
An example of this is Kim Ung-Yong, a Korean boy who spoke four languages and did complicated mathematical problems before he was five years old.
His parents are both university professors. But this does not mean that every university professor will have such a genius for a child. It simply means that this boy inherited a great capacity for learning from his parents and that he was exposed to languages and mathematics when he was very young.
Your brain will stop growing in size when you are about 15 years old.