Boys' brains generally develop somewhat more slowly than girls'. But of course there are individual differences and the natural environment of the young people, in addition to the hereditary factors, has a major influence on these developments.
In a structural sense, boys have slightly larger brains than girls. This has only to do with their somewhat larger body size. The size of the brain is related to the size of the rest of the body. In general, girls have a more robust corpus callosum than boys. The corpus callosum is the thick bundle of white matter fibers that forms the main communication route between the two hemispheres of the brain. The corpus callosum is the highway of information flows. In girls, the corpus callosum has more fibers, reaching to more areas in each hemisphere. Such a structural advantage fits the stereotype of women as being better at multitasking and verbalizing emotional thoughts. Men, on the other hand, have more variation in the structure of their parietal cortex. Parts of the parietal cortex make important contributions to working memory, but the main purpose of the parietal cortex appears to be the processing of spatial information, such as navigation and tracking of moving objects.
Friendship research shows that three-year-olds name just as many girls as boys as boyfriends or girlfriends. At the age of five, that has changed. Then only one fifth of the girls name a boy as a boyfriend and also one fifth of the boys name a girl. At the age of seven, the sexes are virtually separated. Hardly any boy or girl names someone of the opposite sex as a best friend anymore. Those two worlds don't cross paths again until boys and girls start dating each other as adolescents.
In the meantime, boys and girls have very different experiences in dealing with emotions and learn to deal with them differently.
By the age of ten, about an equal percentage of boys and girls are openly aggressive. Only around the thirteenth year does a difference between the two sexes appear. Girls' aggressiveness goes into hiding. They become better than boys at more intelligent tactics, such as death, negative gossip and indirect revenge. Boys are still confrontational and are not aware of more covert, smarter tactics. Boys and later men also develop less sophistication in other emotional areas.
A striking difference occurs when girls play or play sports together. They prefer to do this in small groups with an emphasis on minimizing conflict and maximizing cooperation. Boys play in larger groups with an emphasis on competition. Who is the best, the fastest, can shoot the hardest etc.