Jon Roland's site has posted an interesting discussion of the meaning of "well-regulated." The cites from the Oxford English Dictionary span 1709 - 1894, and suggest "well-regulated" meant something close to "in proper working order."
(I think I've seen a 17th century usages in connection with troops, something about a city being lost to the enemy for want of a body of well-regulated troops in the area, implying that those in the region were ill-disciplined or disorganized). The use of an adjective with "militia" goes back at least to 1625, when Charles I announced he was striving for an "exact militia." To the extent that "militia" meant every man able to bear arms, just having a militia was meaningless ... every nation would have one of those, and "having people of military age is necessary to the security of a free state" might be true but rather obvious.