Conscription laws for front-line Confederate commands extended as high as age 45. Senior reserves (men assigned to rear-area posts) could be conscripted from age 45 to 50. Anyone over the age limit could be discharged. Officers prefered to discharge men whose age and/or health limited their ability to perform actual military service.

Senior reserves were assigned to senior reserve commands. They were not assigned to units like Lay's Cavalry Regiment.

The man you describe might have been assigned to duty with a state militia company, depending on the law in Mississippi. In Alabama men up to age 60 could be required to render service in county militia units. These were local defense companies whose primary mission was preserving law and order at home. The need for this kind of service was far greater than most of us can imagine.