Theodore Roosevelt was the youngest president of USA

Theodore Roosevelt became President at age 42 in 1901 following William McKinley's assassination, making him the youngest person to assume the U.S. presidency. However, John F. Kennedy remains the youngest person ever elected to the presidency, taking office at age 43. The question's wording 'was the youngest president' is technically correct since Roosevelt held the office at a younger age than any other president, though the distinction between assuming office versus being elected matters in precise historical discussions.