There are currently four living ex-Presidents (Ford, Carter, Bush, Clinton). Who was President the first time there were four living ex-Presidents?
John Quincy Adams. From JQ Adams' inauguration until July 4, 1826, Adams, Jefferson, Madison, and Monroe were all alive.
What is the highest number of ex-Presidents to be alive at one time, and when did this happen?
Five, and it happened three times. From Lincoln's inauguration until January 18, 1862, Buchanan, Pierce, Fillmore, Tyler, and Van Buren were all alive. Not surprisingly, Buchanan, Pierce, and Fillmore immediately predated Lincoln as President. However, Tyler and Van Buren were President much earlier but happened to live a while longer. After Tyler were both Polk (who died in 1849) and Taylor, (who died in office in 1850), who did not make it to the Lincoln Presidency.
From Clinton's inauguration until Nixon's death on April 22, 1994, Nixon, Ford, Carter, Reagan, and Bush Sr. were all alive.
From the current Bush's inauguration until Reagan's death on June 5, 2004, Ford, Carter, Reagan, Bush Sr., and Clinton were all alive.
As with the post on Living Ex-Justices, my question for you is: When were there were no living ex-Presidents?
UPDATE: I accidentally listed one of the answers to the second question as an answer to the first question (as some comments correctly pointed out). The answers above are correct as listed, I believe.