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.
Assassinated three years into his term, John F. Kennedy was the youngest at the time of his departure from office (46 years, 177 days); the youngest president to leave the office after 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).
The president born after the greatest number of their successors is John F. Kennedy. He was born after four of his successors: Lyndon Johnson (8 years, 9 months, and 2 days); Ronald Reagan (6 years, 3 months, and 23 days); Richard Nixon (4 years, 4 months, and 16 days); and Gerald Ford (3 years, 10 months, and 15 days). On the other extreme, Ronald Reagan was born before four of his predecessors: Richard Nixon (1 year, 11 months, and 7 days); Gerald Ford (2 years, 5 months, and 8 days); John F. Kennedy (6 years, 3 months, and 23 days); and Jimmy Carter (13 years, 7 months, and 25 days).
The oldest living U.S. president is Jimmy Carter, born October 1, 1924 (age 94 years, 330 days). On March 22, 2019, he also became the nation's longest-lived president, surpassing the lifespan of George H. W. Bush, who died at the age of 94 years, 171 days. Additionally, Carter has the distinction of having the longest post-presidency in U.S. history, currently at 38 years, 219 days. The youngest living president is Barack Obama, born August 4, 1961 (age 58 years, 23 days). The shortest-lived president to have died by natural causes (thereby excluding John F. Kennedy and James A. Garfield, who were both assassinated) was James K. Polk, who died of cholera at the age of 53 years, 225 days; only 103 days after leaving office.