Kids as young as twelve served as drummer boys, but lying about one’s age to recruiters was rampant and recruiters had no ability (and little desire) to verify ages.

Bear in mind that for a good century or two before the Civil War midshipmen (naval officers in training) tended to be children, usually around fifteen but sometimes younger. Kids served and fought on ships and in a handful of emergencies even commanded them. Modern attitudes about the capability, responsibility, and even value of children are just that - modern.