With the assassination of President McKinley, Theodore Roosevelt, not quite 43, became the youngest President in the Nation's history. He brought new excitement and power to the Presidency, as he vigorously led Congress and the American public toward progressive reforms and a strong foreign policy.heodore Roosevelt. With the assassination of President William McKinley, Theodore Roosevelt, not quite 43, became the 26th and youngest President in the Nation's history (1901-1909).