[BlueTrain]

It should be noted that under the original concept of the militia in the United States and before that, in the various colonies, the membership in the militia was not voluntary. It was required. The militia had to army and equip itself and was to muster periodically for training. How all this was managed, I have no idea. It was much later that modifications were made to the system that eventually resulted in the National Guard being created but as others have noted, several states retain something that amounts to a "National Guard Reserve," intended to take the place of the guard when it is mobilized, which has happened a lot lately. There are other functions as well. Virginia has one.

I am distressed, however, to learn that under the original militia act, the upper age limit was only 45 (and I'm in my 60s).

It is probably unique in that the members were (originally) expected to furnish their own arms and equipment. That makes it sound almost medieval, though I don't know if the local lords and squires were usually in command. Other countries having militias, and that's what they're usually called, generally operate on a conscript basis, with arms furnished by the government, even if other equipment or uniforms is lacking. That's how the Boers operated, though the upper age limit was higher than 45.

The concept of the militia was widely accepted in colonial times but it was always considered to be an arm of the civil government, never something independent.