What do you do when you're not president of the United States anymore?
In part, the answer depends on one's age.
The oldest president when leaving office was Ronald Reagan, who gave way to his successor George H.W. Bush on January 20, 1989, when he reached his term limit. Reagan, who was then nearing his 78th birthday, spent the following few years writing a book of memoirs, and engaging in limited public speaking. But after he was diagnosed with Alzheimer's disease in 1994, he lived more or less as an invalid for a period of over nine years, until his 2004 death.
By contrast, the youngest man to leave the presidency alive was Theodore Roosevelt. He was 50 years old when his presidency ended in 1909. I wrote here about his decision not to seek a second full term in 1908, and how he later regretted that decision. He spent a good portion of his time as ex-president attempting to return to the White House. He never achieved that goal; Roosevelt was touted as an early front-runner for the 1920 Republican nomination, but he died on January 6, 1919. While Roosevelt's time as ex-president was more active than Reagan's, the relatively young Roosevelt ironically did not live as long after leaving the White House as Reagan did.
The longest ex-presidency was that of Herbert Hoover. He was 58 years old when he left the White House in 1933, having failed to win reelection the year before. By the time he died in 1964, he had been a former president for more than 31 years. Hoover's reputation eventually recovered from its low point, when he was blamed for the Great Depression during the 1932 campaign. (However, the question of Hoover's culpability is still fiercely debated.) Over the years, he settled in to an elder statesman role. His most high-profile public assignments involved chairing two separate presidential commissions on government reorganization. One of those was appointed by Harry Truman, and the other by Dwight Eisenhower. Those commissions did not seem to have much effect, but those appointments signaled a degree of rehabilitation of Hoover's public standing.
Next: ex-presidents who held public office.