The oldest president to be elected to office was Ronald Reagan, who was 69 when he took office in 1981. Some more interesting facts about the ages of US presidents: The oldest elected president was Reagan (age 69); the youngest was Kennedy (age 43).Theodore Roosevelt, however, was the youngest man to become president—he was 42 when he succeeded McKinley, who had been assassinated.onald Reagan was the oldest president when he was first inaugurated in January of 1981. He was exactly sixty nine years eleven months and fourteen days old. The second oldest president at the time of his inauguration was William Henry Harrison, who was 68 years of age.