From Citizendium, the Citizens' Compendium
Boy : A young male human child, typically older than a baby or toddler, but younger than an adolescent teenager. It's the opposite of girl. The term describes a stage in the life of a male characterized by growth, immaturity, and love of play. In another sense of the word, it describes any male offspring of a mother and father or parent, so an elderly parent can describe his middle-aged son using the expression that's my boy.