The UK entered World War I on the fourteenth birthday of which member of the royal family?

Elizabeth Angela Marguerite Bowes-Lyon (4 August 1900 – 30 March 2002) was the wife of King George VI and the mother of Queen Elizabeth II and Princess Margaret, Countess of Snowdon. She was Queen consort of the United Kingdom and the Dominions from her husband's accession in 1936 until his death in 1952, after which she was known as Queen Elizabeth The Queen Mother, to avoid confusion with her daughter. She was the last Empress consort of India. The Queen Mother was fourteen years of age on the day that the United Kingdom entered World War I.

On 4 August, 1914 Britain (and its Empire) entered the World War with H. H. Asquith of the Liberal Party as British prime minister. Britain's reasons for declaring war were complex: the Treaty of London of 1839 had committed it to safeguard Belgium's neutrality, and the strategic risk posed by German control of the Belgian coast was unacceptable.