I believe the Union Army wanted men 18-45 but would allow boys 16-18 to enlist with parental consent. But both boys and men lied about their age. I am researching the 12th Missouri Cavalry (US), which recruited in the fall of 1863 and early winter of 1864. I found three men who enlisted the same day in Princeton, Missouri, giving their age as 44. This was in September 1863. By June 1864, all three had been discharged on a medical certificate. In each case, the examining doctor gives descriptions like "old age, loss of teeth, and general debility" and says all three were actually 54. One of them was actually closer to 63, based on census and tombstone data. The oldest man I've found in the regiment was 66. He enlisted as age 44 but said the man who enlisted him knew his correct age. He was also discharged in June 1864. (All of these medical discharges occurred after the official organization of the regiment and just before the regiment was shipped off from St. Louis to the war zone in Memphis.)

At this time enlistment was spurred by a $300 bounty--an incentive for lying about your age. Tom Jones