Girls, boys, men and women. Each of these terms is used daily to form groups, set standards, and determine a person's character. These definitions are very much linked to brain development, which is second to actual gender differences.
A boy is a young male who has not reached mental maturity. This includes young baby boys directly after birth to certain teenagers in their late teens, and in some cases their early twenties who have not learned how to exercise good judgment. Boys are generally irrational, acting on primal instincts rather than thought out choices. For boys, the urge to play is a main characteristic.
A girl is a young female who has not reached maturity. This generally applies to baby girls through teen girls. Girls tend to mature into women before boys into men, for reasons of improved mental development. Girls do not differ as much from women as boys do from men, as they do not experience such extreme urges as boys do. Girls are also generally more susceptible to teaching than the other three definitions.
A man is an elderly male who shows common sense, rational thought, and intellectual maturity. The transition into a man general occurs sometime in a male's twenties, be it early or late. Men have a sense of work, and many have the ability and thought to accomplish the necessary work. Men are less motivated by sexual instinct (though still motivated by it), and work towards settling down.
A woman is an elderly female who has reached full intellectual capability and full mental maturity. Girls may tend to become women in their early twenties. Women are full of information and seek to use this. Women are prone to socializing, and have the wish to settle down as opposed to following sexual urges. Women generally have more sense of mind than men. Thus, each term shows the importance of mental development & its role in society.