When I was a boy of fourteen, my father was so ignorant I could hardly stand to have the old man around. But when I got to be twenty-one, I was astonished at how much he had learned in seven years.
~Mark Twain, "Old Times on the Mississippi" Atlantic Monthly, 1874
The quote is attributed to Twain and has his "flavor," but it is apochryphal. It has never been found in his writings by Twain scholars. Also, his father actually died when he was 11.