I want to first note how impressed I am by many of the comments here. Most everyone has hit upon relevant and important points, and discussed them intelligently. The thread is also instructive to me by demonstrating that it fosters better discussion if I could just keep my trap shut. But I can't. It's a mental disorder. I get nervous when I'm not making arguments, or gathering ammunition to make arguments.
Anyway, applicable case law for determining the constitutionality of these laws imposing age restrictions on purchasing and possessing guns is virtually nonexistent, but that which exists is decisive. That's why the NRA's petition challenging the Florida law does not cite a single case (I don't recall another petition in federal court that was void of citation of any case law), and puts forth a freakish Equal Protection rationale about 18-20 year-old women being less likely than males to use semiautomatic guns to commit murders. See:
NRA v Bondi - Complaint
But in 2012, the NRA challenged the constitutionality of the federal law that prohibits firearms dealers from selling handguns to persons younger than 21. The District court granted summary judgment to the government, and the Fifth Circuit panel affirmed. The en banc court declined to review, and the Supreme Court denied cert.
ht
tps://caselaw.findlaw.com/us-5th-circuit/1614698.html
The Circuit court opinion teaches that prohibiting sales of guns to those younger than 21 is one of the "long-standing, presumptively lawful measures" that
Heller explicitly leaves intact. At common law, persons younger than 21 years old were minors, often referred to as "infants," and were universally prohibited from purchasing firearms. Allowing those younger than 21 to buy guns is a fairly recent invention. The Circuit panel meticulously justified the 2-step process of determining constitutionality, as set out in
Heller and which courts have generally employed thence, explaining that, since the law only prohibits sales, it only needs to survive intermediate scrutiny, which it achieves. The opinion further explains that if the law struck at "the core right" identified by
Heller, by prohibiting possession generally, it would need to pass strict scrutiny.
The Florida law referenced in the OP only prohibits sales, thus only requiring intermediate scrutiny, which it easily aces. The Washington state initiative, if it becomes law, would prohibit possession in public, but allows for exercise of Heller's "core right, " i.e., for self-defense in the home. Thus, if strict scrutiny were the appropriate hurdle, the law would seem to clear it.
Heller is sometimes more reasonable than I give it credit for. No one can deny that behind Scalia's iridescent sentences, there was a brilliant mind.