<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 December 13, 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 . The third table lists the instances in which only the living president was the incumbent president, and the fourth lists the instances in which there were six living presidents--five former presidents plus the incumbent--alive at the same time . </P> <P> The median age upon accession to the presidency is roughly 55 years and 6 months . This 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 . The nation's oldest president - elect was Donald Trump, who was inaugurated into office at the age of 7004257880000000000 ♠ 70 years, 220 days . The oldest person to begin a new 4 - year term was Ronald Reagan, who was 7004270120000000000 ♠ 73 years, 349 days at the time of his second inauguration . </P> <P> Assassinated three years into his term, John Kennedy became the youngest at the time of leaving office (7004169780000000000 ♠ 46 years, 177 days); the youngest president to leave office in the course of standard transition was Theodore Roosevelt (7004183900000000000 ♠ 50 years, 128 days). The oldest at the time of leaving office was Ronald Reagan (7004284730000000000 ♠ 77 years, 349 days). </P>

Who is the youngest person to be elected president