• There have been six men who have been the only living United States President (no former President being alive).
• During three periods in United States history, there have been six men alive to have been United States President. 
• Richard Nixon is the only man to have been both an only living United States President (1973–1974) and one of the six living presidents (1993–1994).
• There have been nineteen Presidencies during which a United States President has not died, the most recent being the Presidency of George H.W. Bush.  
• The longest streak of Presidencies without a Presidential death is four, as no United States President died during the terms of Ford, Carter, Reagan and George H.W. Bush.
• Because of his serving 3+ terms, Franklin Delano Roosevelt holds the distinction of serving over the longest period during which no Presidential deaths or changes occurred. 
• The longest period between deaths of United States Presidents was the 26 years, 2 months, and 18 days between the death of George Washington on December 14, 1799 and Thomas Jefferson on July 4, 1826. The shortest were the five hours between the death of Jefferson at around 12:50 PM and the death of John Adams at approximately 6:00 PM on July 4, 1826, which immediately followed the longest.
  Only three times, in 1826, 1862 and 1901, have two United States Presidents died in the same year. 
  • If the longest period between deaths is to be exceeded, no President must die before July 16, 2033 (both the current oldest and the second oldest surviving Presidents, George H. W. Bush and Jimmy Carter, would then be 109 years old).
  • The second longest was the 21 years and 3 months between the deaths of Lyndon B. Johnson on January 22, 1973 and Richard Nixon on April 22, 1994.
• Harry S. Truman died December 26, 1972, just 25 days before Nixon’s second term began. Lyndon Johnson died on January 22, 1973, just two days after Nixon’s second term began. They died only 27 days apart, yet in two different years and two different Presidential terms.
• In order to beat the 6 president record, and assuming that Richard Nixon remains the only president to resign from office, no president can die until at least 2021, assuming that President Barack Obama’s successor serves only one term. 
• Since January 20, 1981 – a period of 32 years, 79 days – there have been at least four presidents living, by far the longest such period in history.

More details from this Wikipedia article.