The youngest authenticated British soldier in World War I was the twelve-year-old Sidney Lewis who fought at the Battle of the Somme in 1916. Lewis's claim has only recently been authenticated. In World War I, a number of young boys joined up to serve as soldiers before they were eighteen, the legal age to serve in the army. Wikipedia*
On the C-SPAN Networks:
Sidney Lewis has one video in the C-SPAN Video Library; the first appearance was a 2001 Forum.