Congress required men ages 18 to 35 to serve. In 1863 limits were extended to age 45, and finally in 1864 young men age 17 and older men ages 45 to 50 were required to serve under limited circumstances. Men ages 45 to 60 had been members of the state militia, ordinarily not in Confederate service.

Most of the particulars you cited will fit this man --
http://history-sites.com/cgi-bin/bbs53x/vacwmb/webbbs_config.pl?read=5662
He appears to be John J. Cumby of Campbell County VA. That's about where where you told us he would be.

Tell your cousin that John J. Cumby was about twenty-six when enlisted. The 1860 census of Virginia doesn't list anyone named John Cumbee or Cumby over age twenty-seven. Perhaps she's confused him with someone else who didn't join the Confederate military.

Copies are available of both his Confederate AND Federal service files. If you're interested, click in the box at the top of this page.