You didn't mention where you looked, so I can't answer your question. Dunbar Rowland's history is the right place to look for history of any Mississippi unit.
Simeon W. Young belonged to Co. "A", 17th Mississippi Cavalry Battalion. It became Co. "A", 9th Mississippi Cavalry Regiment. This command was also called Miller's after its colonel. Evidently Young had been captured and was in prison when his brigade disbanded in Georgia.
Copies of these two service files are available here --