It might have been called the Boys’ War instead of the Civil War – here are the interesting facts

Authorities differ, and statistics bristle in the controversy, but this is the offering of the Photographic History of the Civil War:

More than 2,000,000 Federal soldiers were twenty-one or under (of a total of some 2,700,000)-
More than 1,000,000 were eighteen or under.
About 800,000 were seventeen or under.
About 200,000 were sixteen or under.
About 100,000 were fifteen or under.
Three hundred were thirteen or under-most of these fifers or drummers, but regularly enrolled, and sometimes fighters.
Twenty-five were ten or under.

Other interesting facts:

A study of a million Federal enlistments turned up only 16,000 as old as forty-four, and only 46,000 of twenty-five or more.
Yet by other authorities, the Union armies were made up like this: 30 per cent of men under twenty-one; 30 per cent from twenty-one to twenty-four; 30 per cent from twenty-five to thirty; 10 per cent over thirty.
Confederate figures are skimpier, but one sample of 11,000 men produced about 8,000, the great majority, between eighteen and twenty-nine. There was one of thirteen, and three were fourteen; 31 were fifteen; 200 were sixteen; 366 were seventeen; and about a thousand were eighteen. Almost 1,800 were in their thirties, about 400 in their forties, and 86 in their fifties. One man was seventy, and another, seventy-three.
Most of the youths of tender age slipped in as musicians, for there were places for 40,000 in the Union armies alone. There are numerous tales of buglers too small to climb into saddles unaided, who rode into pistol-and-saber battles with their regiments. Most famous of these on the Union side was Johnny Clem, who became drummer to the 22nd Michigan at eleven, and was soon a mounted orderly on the staff of General George H. Thomas, with the “rank” of lance sergeant.
No one knows the identity of the war’s youngest soldier, but on the Confederate side, in particular, there was a rush of claimants. Some of their tales belong with the war’s epic literature:
George S. Lamkin of Winona, Mississippi, joined Stanford’s Mississippi Battery when he was eleven, and before his twelfth birthday was severely wounded at Shiloh.
T.D. Claiborne, who left Virginia Military Institute at thirteen, in 1861 reportedly became captain of the 18th Virginia that year, and was killed in 1864, at seventeen. (This likely belongs with the war’s apochrypha.)
E.G. Baxter, of Clark County, Kentucky, is recorded as enlisting in Company A, 7th Kentucky Cavalry in June, 1862,when he was not quite thirteen (birth date: September 10, 1849), and a year later was a second lieutenant.
John Bailey Tyler, of D Troop, 1st Maryland Cavalry, born in Frederick, Maryland, in 1849, was twelve when war came. He fought with his regiment until the end, without a wound.
T.G. Bean, of Pickensville, Alabama, was probably the wars most youthful recruiter. He organized two companies at the University of Alabama in 1861, when he was thirteen, though he did not get into service until two years later, when he served as adjutant of the cadet corps taken into the Confederate armies.
M.W. Jewett, of Ivanhoe, Virginia, is said to have been a private in the 59th Virginia at thirteen, serving at Charleston, South Carolina, in Florida, and at the siege of Petersburg.
W.D. Peak, of Oliver Springs, Tennessee, was fourteen when he joined Company A, 26th Tennessee, and Matthew J. McDonald, of Company I, 1st Georgia Cavalry, began service at the same age.
John T. Mason of Fairfax County, Virginia, went through the first battle of Manassas as a “marker” for the files of the 17th Virginia at age fourteen, was soon trained as a midshipman in the tiny Confederate Navy, and was aboard the famed cruiser Shenandoah.
One of Francis Scott Key’s grandsons, Billings Steele, who lived near Annapolis, Maryland, crossed the Potomac to join the rangers of Colonel John S. Mosby, at the age of sixteen.