Who was the oldest U.S. President?

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. THE OLDEST LIVING former president was Gerald Ford, who was born on July 14, 1913, and died on Dec.27, 2006, at age 93. The second oldest was Ronald Reagan, who also lived to be 93 years. ("Presidential Trivia")