No he was not too old to have been in some military unit. "Served" implies some active-duty service, which would apply to the standing armies (State Troops and Continental Regiments) but not always to County Militias.
County Militia units consisted in all able-bodied free white males aged roughly 16-60 (varied by time and place) who were residents of that County. Their purpose was defense within the County. Most militiamen saw no active-duty service, but all were required to attend regular drills and nose-counting. These musters were the occasions for creating many surviving militia rosters, but do not in themselves reflect active-duty service.
Active duty for militiamen might include patrolling, garrisoning a local fort or blockhouse, or assisting escort of supplies bound elsewhere. Some militia units were indeed sent outside the County, even outside the State, for special crises or expeditions, mostly on the frontiers, but this was rare.
A few old men, if still able, were more or less drafted into service, particularly if they had active service as officers during the French and Indian War. A lot depended on local conditions.