Respectfully Quoted: A Dictionary of Quotations. 1989.
Edward Young (16831765)
At thirty, man suspects himself a fool; Knows it at forty, and reforms his plan; At fifty, chides his infamous delay, Pushes his prudent purpose to resolve; In all the magnanimity of thought Resolves, and re-resolves; then dies the same. And why? Because he thinks himself immortal. All men think all men mortal but themselves.
EDWARD YOUNG, Night Thoughts, 1, lines 41724, The Complete Works, Poetry and Prose, ed. James Nichols, p. 13 (1854, reprinted 1968).