42 years, 10 months
Theodore Roosevelt was the youngest president in history. He was also one of the most innovative, brash, and vigorous.
TR was installed as William McKinley's vice president because his competitors wanted to bury Roosevelt in a do-nothing, isolated job. But when President McKinley was assassinated in September 1901, TR became arguably the most influential man in the world.
He moved the White House from the sidelines to the center of American life and was the epitome of the young American leader.
Next: John F. Kennedy