Age was not a hard and fast barrier if the person was a free white able-bodied male and had a horse and/or rifle.In the Mohawk Valley, a man in his 70s with military experience from the French & Indian War took over the Company of one of his sons who had deserted.
In general, all free white able-bodied males between ages roughly 16-60 (varies by time & place) were supposed to be available for County Militia duty, for defense within the County.Even if your man were not the one in the 12th VA, he might have had active-duty Militia service.Some VA militia saw a lot of active duty, some none at all.Some were sent on an emergency basis outside the County, but this was rare and exceptional.
It is also possible that your man sold goods or services to the militia, to State Troops or for use by the Continentals.Claims for reimbursement that were sent to the VA Legislature have been published in booklets arranged by County and in bound volume as /Public Service Claims/; these are available for sale and in major libraries.
In any event it is always difficult to match a particular person with a name on a list.Such a common name presents the research task of very well documenting his particular life-path.This would include looking at all available tax lists, land records, estate records and County Court records for mentions of him in connection with parents, spouse, children, other relatives and neighbors.The last two categories are very helpful as part of "cluster research" that can help distinguish between individuals by the same name.This sort of research requires in-person work in the County Courthouse, the VA State Library, and historical society collections.Little of this sort of material has been published or transcribed by volunteers and posted on the internet.
There are published genealogical guides that can give you a good sense of what to expect from specific types of records.The LDS at www.familysearch.org has a series of research guides arranged by State.