president who was the youngest when he took office
He was the youngest president ever elected to the presidency at the age of 43. However, Theodore Roosevelt was the youngest man to serve as president when he took over after William McKinley's assassination. He was 42. Here are the ages of the ten youngest presidents: 1  Theodore Roosevelt - 42 years, 322 days. 2  John F. Kennedy - 43 years, 426 days. 3  Bill Clinton - 46 years, 154 days.