“What? Was man made a wheel-work to wind up,
And be discharged, and straight wound up anew?
No! grown, his growth lasts; taught, he ne'er forgets:
May learn a thousand things, not twice the same.”

Interview, 2004 http://www.theguardian.com/football/2004/apr/04/sport.features

Source: When the Snow Fell

Sir Anthony Love, Act ii, scene 1; reported in Bartlett's Familiar Quotations, 10th ed. (1919). Compare: "I weigh the man, not his title; 't is not the king's stamp can make the metal better", William Wycherley, The Plaindealer, Act i. scene 1.

Vol. I, Ch. 4, The World As Will: Second Aspect, as translated by Eric F. J. Payne (1958) p. 322
The World as Will and Representation (1819; 1844; 1859)

Though Erdős used this remark, it is said to have originated with his friend Stanisław Ulam, as reported in The Man Who Loved Only Numbers : The Story of Paul Erdős and the Search for Mathematical Truth (1998) by Paul Hoffman
Variants:
The first sign of senility is when a man forgets his theorems. The second sign is when he forgets to zip up. The third sign is when he forgets to zip down.
As quoted in Wonders of Numbers : Adventures in Mathematics, Mind, and Meaning (2002) by Clifford A. Pickover, p. 64
There are three signs of senility. The first sign is that a man forgets his theorems. The second sign is that he forgets to zip up. The third sign is that he forgets to zip down.
Misattributed

Attributed in Paul Hoffman, The Man Who Loved Only Numbers: The Story of Paul Erdős and the Search for Mathematical Truth (1998)
This has also been attributed, with variants, to Paul Erdős, who repeated the remark.

Confessions Of A Sceptic
The Nemesis of Faith (1849)

1920s, Whose Country Is This? (1921)