“Certainly nobody under 21 should have an AR-15”
That’s a specific brand. What about all the others patterned after that brand?
What about AK-47s and rifles patterned after them?
Mini-14’s, etc.?
M1 Carbines, etc.?
and of course raise the voting age to 21.
And if you’re not trustworthy enough to be allowed to buy a firearm to protect your family, raise the marriage age to 21.
10 U.S. Code § 246 - Militia: composition and classes (emphasis added):
(a) The militia of the Uni ted States consists of all able-bodied males at least 17 years of age and, except as provided in section 313 of title 32, under 45 years of age who are, or who have made a declaration of intention to become, citizens of the United States and of female citizens of the United States who are members of the National Guard.
(b) The classes of the militia are—
(1) the organized militia, which consists of the National Guard and the Naval Militia; and
(2) the unorganized militia, which consists of the members of the militia who are not members of the National Guard or the Naval Militia.
We'll need to raise that "17 years of age". If you can't buy yourself a gun you shouldn't be in the militia.