“My experience is that as soon as people are old enough to know better, they don't know anything at all.”

Source: The Shadow of the Wind

From Omni interview, "The Smartest Man in the World" (1979) p. 203
The Pleasure of Finding Things Out (1999)

Mat Buckland, AI Techniques for Game Programming (2002), Cincinnati, OH: Premier Press, 36 ISBN 1-931841-08-X.
Attributed

"How Awful When Poetry Ages As It Is Read"
I've Learned Some Things (2008)
Context: I'd make me into a brand new sailor if I were God
Maybe there were new things over there
It comes from within me to write as though rabid, I'm hungry, do you understand
Let the doctors call it what they will
Who can know anything best of all
What does it mean to know anything best
Which religion doesn't grow old

Source: Bleak House (1852-1853), Chapter 41

Variant: Professor Johnston often said that if you didn't know history, you didn't know anything. You were a leaf that didn't know it was part of a tree.
Source: Timeline

Source: The Member of the Wedding