Go with the first thing you think of
President who lived the longest after leaving office.
My first thought was Carter. Then I started thinking about it. TR died young(ish) but was young when he became president. Adams, maybe? How old was he when he was elected? Reagan - no, he was an old president, forget him.
Man, I have no freaking idea.
And it turns out to be Carter.
How did I know that?