The youngest individual to assume the presidency of the United States was Theodore Roosevelt, who took office at the age of 42. He became president in 1901 after the assassination of William McKinley. Roosevelt's presidency was significant for its progressive reforms, including trust-busting and conservation efforts, marking a shift in the role of the federal government in economic affairs. His energetic leadership style and focus on domestic issues helped shape modern American politics and lay the groundwork for future policy developments. Roosevelt's age upon taking office reflects historical shifts in political norms regarding leadership.