Old minds are like old horses; you must exercise them if you wish to keep them in working order.

John Adams (1735–1826) American lawyer, Founding Father, statesman, US President (1797–1801)

(Attributed)
    (Source)

Attributed by Josiah Quincy III, as described anecdotally in L. Maria Child's essay "Hints About Health," collected in her book Looking Toward Sunset: From Sources Old and New, Original and Selected (1865). Child describes Quincy attributing his own long life and acuity to the above advice given by Adams when Adams was asked how his mind was so vigorous in old age.

At one point I had this exchange sourced as being a letter from Adams to Quincy (1825-02-14). I am at present unable to find a record or reference online of such a letter.

Sometimes (perhaps due to the recipient's name), the quote is misattributed to Adams' son, John Quincy Adams.