I suspect that even after McDonald, most gun controls will be upheld, either on the theory that certain kinds of gun possession are outside the scope of the Second Amendment as interpreted in Heller (e.g., bans on gun possession by felons), or on the theory that the gun control imposes only a slight burden on the right to keep and bear arms for self-defense and thus doesn’t “infringe” that right. (See here for more on that.)
But what about limits on gun possession by 18-to-20-year-olds? New York City totally bars gun possession by 18-to-20-year-olds. Illinois bars gun possession by 18-to-20-year-olds, except with the permission of a parent, and sometimes not even then. Many other states bar handgun possession by 18-to-20-year-olds. See N.Y. Penal Law § 400.00; N.Y. City Admin Code § 10-303; NYPD, Permits | Rifle/Shotgun Permit Information; 430 Ill. Comp. Stat. Ann. §§ 65/2(a)(1), 65/4(a)(2)(i) (barring gun ownership or possession by under-21-year-olds unless they have the written consent of a parent or guardian, and the parent or guardian is not himself disqualified from owning guns, which entirely bars 18-to-20-year-olds from possessing a gun if their parents are dead, or if the living parent or parents are felons, nonimmigrant aliens, mental patients, or otherwise disqualified from owning a gun in Illinois); Conn. Gen. Stat. Ann. §§ 29-34, -36f (banning handgun possession by anyone under 21); N.M. Stat. § 30-7-2.2 (2004) (banning handgun possession by anyone under 18). Federal law doesn’t ban such possession, but it does bar gun dealers from selling handguns to 18-to-20-year-olds, which makes handguns available to 18-to-20-year-olds only by the good graces of a nondealer third party who is willing to sell to them.
As I noted, there’s a possible argument that gun possession by 18-to-20-year-olds is also outside the scope of the Second Amendment as interpreted by Heller, because historically the age of majority has been 21. But I doubt that this would work, because the pre-1970s cases that I’ve seen involving lesser constitutional rights for minors — lesser free speech rights, lesser religious freedom rights, and lesser criminal procedure rights — involved age cutoffs of 18 or less. Whatever setting the age of majority at 21 might have meant for purposes such as contracting, parental authority, and the like, it seems not to have affected those other constitutional protections. So my sense is that these laws might well be struck down, especially if courts take seriously the Court’s suggestion that “incorporation will [not] require judges to assess the costs and benefits of firearms restrictions and thus to make difficult empirical judgments in an area in which they lack expertise” and that instead “[t]he very enumeration of the right takes out of the hands of government — even the Third Branch of Government — the power to decide on a case-by-case basis whether the right is really worth insisting upon.”