The word manhood describes both the adult period in a male human's life, and the qualities that make him a man. Some people believe that a boy reaches manhood when he hits puberty, while others maintain that manhood is based on becoming independent and responsible. Peter Pan just opted out.
Teenage boys sometimes do dangerous things to "prove their manhood," like driving recklessly or getting in fights. It's not uncommon to equate manhood with "macho" characteristics and behavior, but the true proof of manhood lies in things like being a good father or becoming a responsible and caring adult. In the thirteenth century, manhood applied to both men and women, "state of being human," from the Old English man, "person."