The Grand Army of the Republic was an army of very young men. How young it was is shown by the fact that of the men who enlisted to serve in the Union Army, about half of them were under twenty. But 800,000 were seventeen or under and 100,000 were fifteen or younger.
Over 300 boys, thirteen and under, were officially mustered into Federal service. The officers ignored the facts and took the word of the boys as to age, although they suspected the truth. At first they refused to accept nine-year-old John Lincoln Clem, but he succeeded in getting on the muster roll when he was eleven, then an army veteran of two yearss.
When he finally retired from army service in 1915, General Clem was the last man serving his country who had seen action in the Civil War.
Source: "Lincoln's Littlest Soldier," by George C. Crout.
John Lincoln Clem, Arlington National Cemetery Website