“Adulthood does not exist. Man is an eternal child.”

Vorkosigan Saga, A Civil Campaign (1999)
Context: Adulthood isn’t an award they’ll give you for being a good child. You can waste … years, trying to get someone to give that sort of respect to you, as though it were some sort of promotion or raise in pay. If only you do enough, if only you are good enough. No. You have to just … take it. Give it to yourself, I suppose. Say, "I’m sorry you feel like that", and walk away.

Source: Let's Explore Diabetes with Owls

I, q. 10, art. 3, ad 2
Summa Theologica (1265–1274)

"Words Are Easy, Books Are Not," interview with Bob Morris, The New York Times (1994-08-10), Late Edition, Section C, page 1, column 1.

Thoughts and Glimpses (1916-17)

Quoted by Hippolytus, Refutation of all heresies, IX, 9, 4 (Fragment 52), as translated in Reality (1994), by Carl Avren Levenson and Jonathan Westphal, p. 10
Variants:
History is a child building a sand-castle by the sea, and that child is the whole majesty of man’s power in the world.
As quoted in Contemporary Literature in Translation (1976), p. 21
A lifetime is a child playing, playing checkers; the kingdom belongs to a child.
As quoted in The Beginning of All Wisdom: Timeless Advice from the Ancient Greeks (2003) by Steven Stavropoulos, p. 95
Time is a game played beautifully by children.
As quoted in Fragments (2001) translated by Brooks Haxton
Lifetime is a child at play, moving pieces in a game. Kingship belongs to the child.
As quoted in The Art and Thought of Heraclitus (1979) translated by Charles H. Kahn

Source: Eat, Pray, Love

Book 4, pt. 2, v. 12
Variant translations by Lin Yutang:
A great man is one who has not lost the child's heart.
A great man is he who has not lost the heart of a child.
The Mencius

History and Utopia (1960)