Mary Helen --
The Act of April 16, 1862, called for enlistment of most men between the ages of 18 and 35. Those under and older these ages were not required to serve. In this case the soldier received a discharge to go home, knowing that he would need to reenlist when he turned eighteen. Of course a volunteer could choose which unit in which to enlist, and for reasons that only he could explain, he joined a different company.
Muster rolls for most Confederate units are missing for the last two years of the war. Most were either lost or destroyed, but in some cases I wonder if some were ever recorded as they should've been.