I couldn't decide either whether Joe was talking about the last surviving Confederate Veteran by his question or who was the oldest known Confederate soldier that served during the War, like John Burns did during the Battle of Gettysburg. I assumed this latter tract of trying to answer his question.
And while it is assumed that the Confederate Army was full of 'old men' and 'babies" especially during the last of the War, the records do not seem to indicate this even though there were a large number of 'home guard' company in the trenches at Richmond and Petersburg. I was attempting to point out that indeed 55 year old men and 16 year boys in the Confederacy were subject to the Conscription Act by 1865, but, it is hard to prove that there were 10 year old drummer boys or 80 year old privates in the ranks.
Now true, there are several examples of 70 year old plus men enlisting in the first fevered days of the war. But medical examiners quickly discharged those men as being unfit for duty. And certainly even John Burns of Gettysburg fame was 'unfit' for steady military duty. And that is what I believe to be the key element here is the fitness of such men to fulfill the harsh duty of military life during that war.
For that example, and you are well aware of this yourself, we only have to look at re-enacting today to see that reality. That is also the reason that men 50 to 55 were allowed to render their service with State Troops. That is why I commented as to what a persons definition of service was for certainly the less demanding service as State Troops allowed for the enlistment of men who were not otherwise fit for regular service. But it was service non the less. And that would include men of greater age than 55. Yet I don't know of many State Troops records that have survived or researched.
Me personally, I have had to join the 'Invalid Corps' a long time ago, as far as my service in the field goes.