Professor Volokh takes a look at the issue of under-21 possession on possessing any firearm in New York City.
Would the right to keep and bear arms not fully apply to under-21-year-olds, the way some constitutional rights today don’t fully apply to under-18-year-olds (consider the right to sexual autonomy, the right to marry, the right to abortion, which could be limited through certain kinds of parental consent laws, and likely the right to bear arms itself)? Or does the right apply to all adult citizens — unless otherwise disqualified by reason of felony conviction or the like — under today’s age of majority, regardless of what the age of majority was at the time? Or has the right always extended to everyone 18 and above, regardless of the age of majority for other purposes?
I’m going to guess, as with most rights, some restrictions will be permitted while others will not. At the very least, I would imagine it would be unconstitutional to ban all juvenile possession of arms, for any purposes.  But it’s a good question to start asking.
This is really dangerous. This allows the state to withhold rights of adult citizens based upon “age” requirements that are not specifically prescribed in the constitution. People need to see this as a seriously damaging power grab.
Restrictions by age aren’t exactly new. For example, from the Alabama criminal code:
13A-11-57. Selling, etc., pistol or bowie knife to minor.
Any person who sells, gives or lends to any minor any pistol or bowie knife, or other knife of like kind or description, shall, on conviction, be fined not less than $50.00 nor more than $500.00.
18 year olds are able to serve in the armed forces. it goes without saying that they should be able to purchase firearms for their own use. In my opinion, any law that prevents that should be unconstitional.
I’ll quote from an ancient bit of U.S. code “The militia of the United States consists of all able-bodied males at least 17 years of age….”
http://www.law.cornell.edu/uscode/10/311.html
The constitution was amended (#26) to allow 18-year-olds to vote because if they could be drafted to involuntarily serve in a war, they should have a say in who sends them. The idea that at the age of 18, you can buy cigarettes, operate heavy machinery, buy a home, serve in the military, vote, be given the death penalty in some states, participate in pornography, and have an abortion, but you can’t be trusted to own a gun or drink a beer is preposterous.
The court cases will follow the argument made in abortion rights cases, that being a minor should not impede the right.
The result of the court cases will be that an enumerated right can have an age limit, with no judicial or administrative route around it, but maybe parental oversight allowing the exercise of the right.The case in most states for abortion is that minors can have one with parental consent or with a rather easy, and anonymous, review by a court. I don’t expect any courts to rewrite the abortion situation, and if the 2nd Amendment situation ends up different, there is a very good case for review up to the Supremes, where one or the other will change to make both match.
Just my best guess.
I thought about the possibility that the government could claim an interest in ensuring supervised use of a firearm, but since the firearm’s quintessential use is self-defense, can I hand my kid a PPK to carry around as long as he’s in my presence, and under my supervision? Can the government restrict that? There’s probably a good case for no.