You've heard about boys joining the army in the Civil War who were so young that they weren't given weapons but played a drum instead. Perhaps you saw the 1963 Disney TV movie Johnny Shiloh. The film was fictionalized, but was based on a real boy who ran away from home to join the Union Army in 1861 when he was only nine years old.
John Lincoln Clem was not accepted as a soldier at age nine, but he refused to go home and followed the 22nd Michigan Infantry Regiment. They had to feed the child, and the officers chipped in to pay him. The army officially accepted his enlistment a couple of years later. He was a drummer boy, but he also learned to shoot, and for his heroic deeds in the Battle of Chickamauga, he was promoted to sergeant. Johnny Clem was (and still is) the youngest soldier ever to become a noncommissioned officer in the US Army. He was twelve years old.
After the war, in which Clem was wounded twice and captured once, he left the army to go to high school. Then he rejoined in 1871 when U.S. Grant appointed him a second lieutenant. Clem retired as a brigadier general in 1915, the last Civil War soldier on active duty. On retirement, he was promoted to major general. Read more on Clem's adventures and accomplishments at Wikipedia. -Thanks, WTM!