<P> This is a list of presidents of the United States by age . The first table charts the age of each United States president at the time of presidential inauguration (first inauguration if elected to multiple and consecutive terms), upon leaving office, and at the time of death . Each president's lifespan (age at death) is measured in two ways, to allow for the differing number of leap days that each experienced . The first figure is the number of days between the president's date of birth and date of death, allowing for leap days; in parentheses, the same period is given in years and days, with the years being the number of whole years that the president lived, and the days being the number of days since the last birthday . Where the president is still living, their lifespan is calculated up to April 9, 2018 . The second table includes those presidents who had the distinction among their peers of being the oldest living president, and charts both when they became and ceased to be oldest living . </P> <P> The median age upon accession to the presidency is 55 years and 3 months . This is how old Lyndon B. Johnson was at the time of his inauguration . The youngest person to assume the office was Theodore Roosevelt, who became president at the age of 42 years, 322 days, following William McKinley's assassination; the oldest was Donald Trump, who was 70 years, 220 days old at his inauguration . The youngest person to be elected president was John F. Kennedy, at 43 years, 163 days of age on election day; the oldest was Ronald Reagan, who was 73 years, 274 days old at the time of his election to a second term . </P> <P> Assassinated three years into his term, John F. Kennedy was the youngest at the time of leaving office (46 years, 177 days); the youngest president to leave office at the conclusion of a normal transition was Theodore Roosevelt (50 years, 128 days). The oldest at the time of leaving office was Ronald Reagan (77 years, 349 days). </P>

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