“There are many things in this world that a child must not ask about.”

As quoted by James Baldwin, “Highroad to Destiny,” a chapter in Martin Luther King, Jr.: A Profile, edited by C. Eric Lincoln, New York, NY, Hill & Wang, 1993, p. 97, (Rev. King speech to a black congregation in St. Louis), reprinted from the February, 1961 issue of Harper’s magazine under the title: “The Dangerous Road Before Martin Luther King.”
1960s

Original: (it) Quando si è bimbi si fanno tante domande pretendendo altrettante risposte. Quando si è adulti si evitano molte domande, per evitare inutili risposte.
Source: prevale.net

And that last bit I think often gets missed out.
Interview at the Academy of Achievement (23 May 1998).

As quoted in Extraordinary Lives: The Art and Craft of American Biography (1986) by Robert A. Caro and William Knowlton Zinsser. Also quoted in Truman by David McCullough (1992), p. 44, New York: Simon & Schuster.-
Context: You must be frank with the world; frankness is the child of honesty and courage. Say just what you mean to do on every occasion, and take it for granted you mean to do right … Never do anything wrong to make a friend or keep one; the man who requires you to do so, is dearly purchased at a sacrifice. Deal kindly, but firmly with all your classmates; you will find it the policy which wears best. Above all do not appear to others what you are not.

Beyond Natural Resources (2005)

Source: When the Snow Fell

Source: The Man in the High Castle (1962)
Context: When I was a child, I thought as a child. But now I have put away childish things.... I must be scientific.

Source: Bitter Angels (2009), Chapter 2 (p. 18)