Are you now as sharp mentally as you were years ago?
When I was young, mathematics was the passion of my life. The same passion has returned upon me [at age 70], but with unequal powers. Processes which I then read off with the facility of common discourse, now cost me labor, time, and slow investigation.
Thomas Jefferson to William Duane, 1812, 193
Patrick Lee’s Explanation
Three years retired from the Presidency, Jefferson now had time for both scholarly and leisurely pursuits. He was able to take up interests that he had to lay aside decades before. But he found his brain didn’t work nearly so well at 70 as it did when he was 20!