Twenty-fifth Amendment to the United States Constitution
On November 22, 1963, President John F. Kennedy was assassinated.  Kennedy's assassination made it very clear to Congress that they needed to figure out a solution about presidential succession right away.  The United States was in the middle of the Cold War.  The new President, Lyndon B. Johnson, had previously had a heart attack in 1955 along with a family history of Johnson men dying relatively young (in their early 60s, which he did in 1973 aged 64). The next two people in line for the presidency were the Speaker of the House of Representatives, John McCormack (who was 71 years old), and the President pro tempore of the Senate, Carl Hayden (who was 86 years old).  Congress started moving more quickly.