Some interesting Presidential Statistics on Presidents:
Specifically, the four times we have had six Presidents alive at the same time and also six times with no President alive other than that one serving? So here goes:
Six Presidents alive:
March 1861 to January 1862—Martin Van Buren, John Tyler, Millard Fillmore, Franklin Pierce, James Buchanan, Abraham Lincoln —320 days until death of Tyler.
January 1993 to April 1994—Richard Nixon, Gerald Ford, Jimmy Carter, Ronald Reagan, George H. W. Bush, Bill Clinton—1 year 92 days until death of Nixon.
January 2001 to June 2004—Gerald Ford, Jimmy Carter, Ronald Reagan, George H. W. Bush, Bill Clinton, George W. Bush—3 years 137 days until death of Reagan.
January 2017 to November 2018—Jimmy Carter, George H. W. Bush, Bill Clinton, George W. Bush, Barack Obama, Donald Trump—1 year 314 days until death of H. W. Bush.
Six living Presidents, no living predecessors:
April 1789 to March 1797—George Washington, no earlier President
December 1799 to March 1801—John Adams after death of Washington
July 1875 to March 1877—Ulysses S. Grant after death of Andrew Johnson
June 1908 to March 1909—Theodore Roosevelt after death of Grover Cleveland
January 1933 to March 1933—Herbert Hoover after death of Calvin Coolidge
January 1973 to August 1974—Richard Nixon after death of Lyndon B. Johnson