He that would pass the latter part of life with honour and decency, must, when he is young, consider that he shall one day be old; and remember, when he is old, that he has once been young.

Samuel Johnson (1709–1784) English writer, lexicographer, critic

Essay (1750-09-08), The Rambler, No. 50
    (Source)

Often misattributed to Joseph Addison.