On September 14, 1901, Theodore Roosevelt was sworn in as the youngest man to serve as US President after William McKinley died after being shot by an assassin. An excerpt from the article:
"Theodore Roosevelt was governor of New York before becoming U.S. vice president. At age 42, Roosevelt became the youngest man to assume the U.S. presidency after President William McKinley was assassinated in 1901. He won a second term in 1904. Known for his anti-monopoly policies and ecological conservationism, Roosevelt won the Nobel Peace Prize for his part in ending the Russo-Japanese War."