The youngest president in U.S. history was Theodore Roosevelt, who became president at 42 years and 322 days old after the assassination of President William McKinley in 1901. The youngest president to be elected was John F. Kennedy, who was elected at 43 years and 236 days old. These two presidents are famously recognized as the youngest individuals to assume the U.S. presidency, with Roosevelt being the youngest overall due to succession and Kennedy being the youngest elected president.