The youngest president in U.S. history was Theodore Roosevelt. He ascended to the presidency at the age of 42 in 1901, following the assassination of President William McKinley. Roosevelt's presidency marked significant changes in American domestic and foreign policy, including progressive reforms and initiatives like trust-busting and conservation of natural resources. His energetic and charismatic leadership style made him a popular figure. Roosevelt's age at the time of his inauguration set the record, which stands, demonstrating how his youthful vigor contributed to a transformative period in U.S. history and governance.