"The City’s primary defense of the age-18 limitation is to argue that minors have no Second Amendment rights at all. To support this sweeping claim, the City points to some nineteenth-century state laws prohibiting firearm possession by minors and prohibiting firearm sales to minors. Laws of this nature might properly inform the question whether minors have a general right, protected by the Second Amendment, to purchase or possess firearms. But they have little relevance to the issue at hand."