Youngest US president to assume office
Theodore Roosevelt
42,322 year(s):day(s)
United States ()

Theodore "Teddy" Roosevelt (USA, b. 27 October 1858) was 42 years 322 days old when he became the 26th US president on 14 September 1901. He assumed the presidency after the assassination of his predecessor, William McKinley, who was killed on 6 September 1901 at the Pan-American Exposition in Buffalo, New York, USA. Roosevelt served two terms as the nation's leader (1901–09).

Among his many achievements, Roosevelt was awarded the Nobel Peace Prize for mediating in the Russo-Japanese War – although this proved controversial, as he also promoted an aggressive US foreign policy. He also made great strides in conservationism, enlarging the national forests in the west of the country, promoted irrigation projects and reserved areas of land for public use. Roosevelt is widely considered to be the first "modern" US president, in that he considerably broadened the importance and strength of the executive office (a US president's immediate staff and support staff). During his tenure, the role of the president (rather than Congress or political parties) became the linchpin of US politics.