Many, including many US citizens, are surprised to learn that Theodore Roosevelt, who served as US president from 1901 to 1909, was the youngest US president in US history in terms of age at the time of being sworn into office. Roosevelt was only 42 when he took the oath of office after the assassination of President William McKinley in 1901. In 1905, he was elected and sworn in for a full term in his own right. This is in contrast to John Fitzgerald Kennedy, who is the youngest US president to take the oath after being elected to the position. JFK was 43 at the time of his inauguration. Both of these presidents share the distinction of promoting ideas that were considered progressive in their day. The oldest US president to ever take office as of 2011 was Ronald Reagan, who was 69 at the time of his inauguration in 1981.
Theodore Roosevelt as a National Leader: