As a matter of law, age is a strange category where some discrimination by government on the basis of age is allowed while other applications are not. What complicates matters is that states have the general police power to regulate for the health, safety and welfare of its citizens, the federal government does not.
For example, legal age of contract in all the states is 18. For that reason, A state probably could not criminalize private consensual sex between those of 18 but below age of 21.
The odd thing about alcohol regulation is that the federal government doesn't legally set the drinking age--e.g. it simply cuts off highway funds from states that set their state laws to below 21. States can and did (Louisiana comes to mind) set their drinking age from 18-21--they just lost federal funds if they set it below. The 21st Amendment repealing prohibition also grants states considerable power to regulate alcohol.
Regarding firearms, we have the strange juxtaposition of already constitutionally tested language on handgun sales limited to those of 21 under Congressional commerce clause power. However, states can and have allowed possession of handguns below 21 (not sales of these) which is a police power exercised by the state. Same thing for long arms, states probably could (depending on their constitution) limit possession and sale of firearms to those below 21.
I suspect that Caro is using that chain of reasoning to declare that the federal government has the power to ban sales of long arms to those below 21 based on stare decisis. However, I am not so sure--it depends on whether the federal courts treat the right to self defense, which is mentioned in Heller and MacDonald, as being a right that would be affected by a ban on sales of long arms. I doubt that the federal courts would hold that the government has the power to criminalize possession of long arms to for adults from 18-21 years old directly under existing precedent, law, and custom. (This is apart from firearms used to facilitate a federal crime where the power of the federal government does exist.) A considerable number of 18-21 year old legal adults could constitute a class where they could not exercise the right of self defense unless access existed to some form of firearms. Many of these are independent of their parents.
If the Supremes treat the 2nd like the contracting rights of 18 year olds, then federal laws might be held unconstitutional. If they treat it as something non-essential like alcohol, then SCOTUS might allow it. It might hinge on the vagaries of Justice Kennedy and Chief Justice Roberts. I suspect that the conservative bloc of Alito, Thomas, and now Gorsuch, are not forcing 2A issues because of the wobbly Chief Justice and the unpredictable Justice Kennedy. If Kennedy or a liberal justice retires, then you might see movement.
