Oldest I bought and shoot regular is a Hall 1819 Flintlock. Took it our corn cruncher hunting some years back but have yet to squeeze the trigger on a deer. Dad got me collecting U.S. military arms many years back. He collected only 1846 muskets and 1858 Remington revolvers. But I expanded.
We do have a few older but they do not get out hunting much.
Oldest would be a flint fouler that has parts on it from around 1760 and possible assembled about 1770. That info is what a builder in Sidell ILL told me in his shop where the smooth bore barrel was deemed unsafe to shoot as it is badly / deeply pitted about 14" down from the muzzle.