I believe it stated out 16, but the Conscription Act of 1882 (may not be correct title) stated 18 and 45. The key was some one had to say you were to young or too old. If you were 15 and a man no one said anything. If you were 48 or 50 and could still carry your load, again no one said anything. But older brothers and kin would comment on both because of the nature of war. Young soldiers followed the camp till they came of age or went home till they were of age. Later in the war ages were adjusted, to get more troops. A lot of the older soldiers signed on as wagoners or contractors of specific trades such as wheel wrights and saddle makers, blacksmiths and other trades were often trades that were short, because of the work loads. These were jobs given to black soldiers early on, but were given to disabled later on. A man with one leg or arm desired the work. Another factor to think about were the late enlistments. Soldiers who joined in 1863 and 1864. Often they were not ask their age. I think the key was ability to function as a soldier. As to 80 year old Militia. In Texas the very young and old were the close in protection of the towns and settlements. Those of age plus or minus a few years were in the state troops to protect against Indians, deserters, gangs, etc., and the few Yankee excursion that did occur.Even with this a lot of places disappeared till after the war and some never came back.

A rough guess, born before1847 and after 1815. There were still a few who would exceed these dates. When I walk around some of the old cemeteries, it is difficult. Now I have a tablet and can do a quick search of the soldier sailors record site at the National Park Service. Not perfect, but on the way.

One of my Georgia kin joined in 1862 when his son died of disease. He was 45 and fought with the 50th Georgia Infantry, through the 4 month Atlanta campaign, the next two months of confusion, and then went with Gen Hood to Tennessee. Some how he was paroled in North Carolina. Born in 1815 and was 48 after 3 years of the most difficult war. He died in 1909. Civil War veterans were still dying in the 1930s, and a few after that!