Georg Christoph Lichtenberg, The Waste Books, tr. R. J. Hollingdale, New York Review Books, 1990, p. 223, Notebook L, Aph. #67:
If we did not remember our youth, we should [would] not be aware of old age: the malady of age consists solely in our no longer being able to do what we could do formerly. For the old man is certainly as perfect a creature in his own way as is the young.
The title I supplied alludes to Paul Simon's Still Crazy After All These Years. (What a great song!)