I found this story on Futility Closet's blog here.
"John Clem was only 10 years old when he ran away to win the Civil War. ('It was necessary that the Union should be preserved,' he later wrote, 'and my help was obviously needed.') Rejected by Michigan’s 22nd infantry regiment, he tagged along anyway as a drummer boy and rode into Chickamauga seated on a caisson and carrying a musket sawed off to match his size. When a Confederate colonel rode up and yelled 'Surrender, you damned little Yankee!' Clem shot him, winning instant fame as 'the drummer boy of Chickamauga.'
"He went on to fight at Perrysville, Murfreesboro, Kennesaw Mountain, Peach Tree Creek, and Atlanta. After the war Ulysses Grant, now president, appointed him second lieutenant; when he retired as a brigadier general in 1915 he was the only Civil War veteran still on duty. 'There is no more popular officer in the army,' wrote the New York Times. 'Colonel Clem is the son-in-law of a Confederate veteran, and this fact he often cites in conversation with friends as proof of the fact that he is the most ‘united American’ extant.' He died in San Antonio in 1937."