Almost as old, the Second Militia Act: The second Act, passed May 8, 1792, provided for the organization of the state militias. It conscripted every "free able-bodied white male citizen" between the ages of 18 and 45 into a local militia company. (This was later expanded to all males, regardless of race, between the ages 18-54)
Those assigned to crew served weapons in modern military units often just have handguns as their personal weapons, e.g. machinegunners.
BTW, I think the NRA is correct and Congress did not have the authority to designate the age distinction for “keep and bear” unless expressly stated otherwise. They would have to redefine what constitutes the militia, and as long as they allow soldiers at the age of 18, they’d have a heck of a time doing that and resolving “equal protection.”