Quoted in Reader's Digest, Sept. 1949. According to Nigel Rees, Cassell's Humorous Quotations, the following appeared in Punch in 1924: " 'It took me nearly ten years to learn that I couldn't write.' 'I suppose you gave it up then?' 'Oh, no! By that time I had a reputation established.' " The issue referred to by Rees is 6 Feb., and the cartoonist is R. Curry.
William Wordsworth, Dorothy Wordsworth, Ernest De Selincourt, Alan G. Hill, Chester Linn Shaver (1967). “The Letters of William and Dorothy Wordsworth: Volume VIII. A Supplement of New Letters”, p.51, Oxford University Press on Demand