You have Edmund Ruffin who in 1861 was 66 years of age when he supposedly fired the first shot at Fort Sumter. This is not actually true and I don't believe that he actually was any "Official" member of any Unit.
On the other hand by 1865 any male between the ages 16 to 55 were subject to conscription in the south. But in the extreme cases young boys between the ages of 16 to 18 and old men between the ages of 50 to 55 were allowed to serve in State troops units instead of the regular Confederate army. However I am sure that there were men much older than 55 who served in State Troop and Militia Units in the south.