I wonder if yet younger boys served "unofficially". The following is from the Battle of Seven Pines on May 31, 1862.

Colonel James Kemper marched up in support of Rodes’ mangled ranks. As Kemper’s men approached the battlefield, they came upon a demoralizing sight: “Long streams of wounded made their appearance on their way back to the rear, in every species of mutilation,” wrote Private Hunter of the 17th Virginia. “Some were borne on stretchers, others swung in blankets from whose folds blood and gore dropped in horrible exudations, staining the ground and crimsoning the budding grass.” Farther along, the troops of the 17th Virginia passed a wrecked Confederate battery whose gunners and horses all lay dead and wounded except for a little boy, the “powder monkey”, who had somehow escaped injury. Private Hunter remembered the boy: “He cowered behind a wheel of one of the guns, with eyes protruding, hands clasped, teeth clenched and a face wearing a look of horrified fright - face so white, so startling in its terror, that it haunted me for days after.”