Good question. Article at link has some intresting points.
Others have said
One might define adulthood as the age at which a person learns he must die and accepts his sentence undismayed.
A competent and self-confident person (an adult Ed.) is incapable of jealousy in anything. Jealousy is invariably a symptom of neurotic insecurity.
And to paraphrase RAH
A man should be able to change a diaper, plan an invasion, butcher a hog, conn a ship, design a building, write a sonnet, balance accounts, build a wall, set a bone, comfort the dying, take orders, give orders, cooperate, act alone, solve equations, analyze a new problem, pitch manure, program a computer, cook a tasty meal, fight efficiently, die gallantly.
So, Happy Father’s Day to one and all.
I like that list.