Earlier quoted context omitted.
In the 18th century the phrase "Well Regulated" had a different meaning, for example a "well regulated clock". It's a misconception that it refers to regulation as we understand it in a modern sense.
If mememory serves me, everybody 16 (18?) and up was "the militia" by default.
"10 USC Ch. 12: THE MILITIA" https://uscode.house.gov/view.xhtml?path=/prelim@title10/sub...