Who was the youngest US president?
List of presidents of the United States by age
Theodore Roosevelt
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 42years, 322days, following William McKinley's assassination; the oldest was Donald Trump, who was 70years, 220days old at his inauguration. The youngest person to be elected president was John F. Kennedy, at 43years, 163days of age on election day; the oldest was Ronald Reagan, who was 73years, 274days old at the time of his election to a second term.