Florida recently enacted a gunowner-control law that included an age restriction. It also banned bump stocks (which I've posted about here, here and here), established a 3 day waiting period, instituted a variant of Gun Violence Restraining Orders (which I've written about here) and expands confiscation powers under the Baker Act which deals with involuntary psychiatric evaluation (this Politifact piece has some interesting information about gun owners and the Baker act). The NRA has filed suit over the age restriction. I do not know what the particulars are of their filing, but I see a clear statutory basis to establish that a state may not restrict firearms to those 21 or over.
Congress, pursuant to Article 1, Section 8 of the u.S. Constitution, has the power to, among other things, call up the militia.
10 U.S.Code § 246 defines the militia as all able bodied males between the ages of 17 and 45 (with noted exceptions).
Looking at this chart if the number of people 21 and over is subtracted from the number of people 18 and over that leaves roughly 14 million people between the ages of 18 and 20. Out of roughly (again doing simple math from the same chart) 100 million folks between the ages of 18 and 45, 14% is a noticeable number.
By barring ownership of long guns to people between the ages of 18 and 20, a state would deprive the u.S. government of a decent chunk of its militia.
(Also, by barring ownership of long guns to people between the ages of 18 and
20, the u.S. government would deprive a state of a decent chunk of its
As long as the federal government has defined, by statute, that 17 to 20 year olds are included as a component of the unorganized militia, then a state may not restrict possession or purchase of long guns. Since militia are expected to provide their own equipment, including personal weapons, then such a state law would interfere with Congress' ability to summon the militia in times of peril. (Washington, D.C. did just that prior to the War of 1812, with disastrous consequences.)
(I also would turn around and file suit against the Feds, arguing that the federal laws preventing purchase of long arms by 17 year olds, and preventing purchase of handguns by those between the ages of 17 to 20, hamper a state's ability to call up the militia in a crisis.)
So if I were in the NRA's place, I'd toss that in there, not only because it is persuasive, but to get the anti-gunners off my back about ignoring the militia component of the 2nd article of the Bill of Rights.