Why did CSA/USA not set the lower limit at 16 since many militia laws at one time had 16 has the lower age limit. And the upper limit at 60 for same reason.
Relative to the Confederacy, it didn't have to, because its States and their troops already did, etc.
Besides the Confederate conscription, the States enrolled men generally from 16 to 60, not otherwise conscripted, into their Militia/State troops. Mississippi commenced such enrollment in 1862. Georgia commenced this enrollment in 1863. Same in the other States, including South Carolina:
So did Alabama, which extended the conscription to the whole of that age group:
In October, Jefferson Davis toured Mobile, and reviewed the cradle and grave troops raised, to which he observed...
Confederate Minister to Europe, John Slidell observed... in explanation of why some Europeans subjects in the Southern States were being conscripted that it was an honest error...
And once enrolled in the militia/home guards/State troops, the young and old were frequently ordered to join the Army, etc. Charles Morse of the Union army noted among the prisoners taken in late 1862 were noticeable numbers of boys and old men among the conscripts:
"You have no idea what innocent, inoffensive men most of them seem to be: a great many are mere boys; there are some old men, too, which humped backs. Scarcely any of them seem to have any idea of what they are fighting for, and they were almost all forced into the army. I talked to one poor little fellow from Georgia who had received a severe wound; he could not have been more than sixteen years old..."
The 1st Ohio Cavalry noted in February, 1863 the Confederate conscription was taking those 15 and up in Virginia:
Lt. Col. E.D. Blake, a Confederate conscription camp officer, in a late 1863 report regarding Virginia forces, noted the soldiers had been recruited in some measure from lads 15 years up (well under the general
legal military age), but that there was no point in adjusting, as he figured after three years of war, nearly all of such were either turning 18 or over or would soon be nearing that age:
The US Army noticed the general age groups' representation in the Confederate ranks. The Army and Navy Journal noted in February, 1864...
The "impaired efficiency" of too many of the boys and old men was noted by many. The Confederate Conscription acts, however, did not limit the conscripts to "able bodied" men, so they were all fair game to be sent to the front.
The 1864 edition of the Southern Manual of Military Surgery confirmed that the conscripts were too frequently disabled, sickly, or weak...
Another Surgeon noted the fresh collections of boys and old men into the Conscripted reserves, etc. were quickly set upon by camp diseases like Typhoid, etc.
A federal cavalryman in October, 1863 observed of several hundred Confederate prisoners, that the mere boys among them seemed particularly wasted by exertion...
The dearth of able-bodied men sent forward by the conscription was complained of by the Army. General Bragg and the generals of the Army of Tennessee begged in the summer of 1863 that the large number of able-bodied military aged exempts should be sent to the front forthwith:
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A year later, General Lee was yet complaining that the conscription was not benefiting the army... Confederate War Department clerk J.B. Jones observed in late 1864:
The exempts and "detailed" men, called variously by Southerners "carpet knights," "bomb proofs" or "buttermilk rangers" etc., were not just given a pass for wealth or slave-holding. William Watson of the 3rd Louisiana noted many had hoped the conscription acts would eventually net the "secessionists" or "political loafers" but such hopes were not realized, as they too were largely exempted by the powers that were...
It is true that the Confederate conscription, officially, eventually declared all men 17 to 50 were to serve (unless exempted), but it would be well to notice even the Confederates were aware the constitution and the laws of the Confederacy were rather more attended to in the breech than otherwise. Basil L. Gildersleeve noticed there was no consistency in the Confederate process...
On September 2, 1864, General Lee himself noted to President Davis the inconsistencies of the conscription act enforcement; keeping the exempts exempted and detailed as far from the front as possible, and pressing harder on
everyone else placed in active service...
In the Northern States, the National Militia laws required 18 to 45 year olds subject to calling forth by the USA. And the "national draft" after July, 1863, required all such to enroll for potential active service (the national enrollment also aiding the States' militia enrollment). Nationally drafted men might pay a commutation to skip a draft, or send a substitute into active service (if he was found able-bodied, etc.). By the end of the war there were 2.2 million enrolled men subject to draft for active service. So no need to extend the calling forth age below 18 or over 45:
Large numbers of lads under 18 and even some men over 45 enlisted and served in the Volunteer Units of the US Army during the war.