<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 November 14, 2017 . 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 roughly 55 years and 6 months, which is about how old Benjamin Harrison was at the time of his inauguration . The youngest person to assume the office was Theodore Roosevelt, who became president at the age of 7004156620000000000 ♠ 42 years, 322 days, following William McKinley's assassination . The youngest person elected president was John F. Kennedy, who was inaugurated into office at the age of 7004159420000000000 ♠ 43 years, 236 days . Assassinated three years into his term, he became the youngest at the time of leaving office (7004169780000000000 ♠ 46 years, 177 days); the youngest person at the time of leaving office after serving a full four - year term was Theodore Roosevelt (7004183900000000000 ♠ 50 years, 128 days). The oldest person at the time of entering office was Donald Trump, at the age of 7004257880000000000 ♠ 70 years, 220 days; Ronald Reagan was the oldest person in office, at the age of 7004284730000000000 ♠ 77 years, 349 days when his presidency ended in January 1989 . </P> <P> The oldest living U.S. president is George H.W. Bush, born June 12, 1924 (age 7004341230000000000 ♠ 93 years, 155 days). The second oldest, Jimmy Carter, has the distinction of having the longest post-presidency in U.S. history, currently at 7004134470000000000 ♠ 36 years, 298 days . The youngest living president is Barack Obama, born August 4, 1961 (age 7004205560000000000 ♠ 56 years, 102 days). </P>

Who was the oldest president when he took office