Citigroup announced that it will "require new retail sector clients or partners to adhere to .... restrict the sale of firearms for individuals under 21 years of age." But what if the retailer is in one of the states that generally ban age discrimination in public accommodations, and include 18-to-20-year-olds in that ban? (Some states only ban age discrimination against those 21 and above, or 40 and above; let's set those aside.)
Any credit card companies that have such policies must exempt retailers' actions in those no-age-discrimination states. When a law bans some action, it also usually (explicitly or implicitly) bans others from trying to cause that action. It's a crime for you to kill someone (without adequate justification), so it's a crime for me to try to pressure you into killing him. It's a tort for you to libel someone (assuming you know the factual allegations you're making are false and defamatory), so it's a tort for me to offer you money to do so.
Likewise, generally speaking, for antidiscrimination law. Consider, for instance, Michigan law; the Elliott-Larsen Civil Rights Act (Mich. Comp. Laws. §§ 37.2301-.2304) bans age discrimination in retail sales, and § 37.2701 likewise provides that no person shall "[a]id, abet, incite, compel, or coerce a person to engage in a violation of this act" or "[w]illfully obstruct or prevent a person from complying with this act" or "interfere with a person in the exercise or enjoyment of ... any right granted or protected by this act." If a credit card company demands that stores illegally discriminate, then it's inciting, compeling, and coercing violations within the meaning of the law, obstructing the stores' complying with the law, and interfering with 18-to-20-year-olds' enjoyment of rights granted by the law.
If a credit card company required its business partners not to sell wedding cakes to gays and lesbians, that would violate the law in those states that ban sexual orientation discrimination by such businesses -- and the ban would apply not just to the cake shops, but also to the credit card company. The same is true for age discrimination in gun sales; the legal analysis is the same (whatever you might think about the moral issues).
Now this is just a matter of state law. If a state bans rifle and shotgun sales to 18-to-20-year-olds, then of course stores would be required to discriminate against them. (Federal law already bans handgun sales by gun dealers to under-21-year-olds.) Likewise, if a state wanted to exempt gun sales from the age discrimination ban, it could do so (though I know of no states that do). But if a state bans age discrimination in gun sales -- as several states (likely about a dozen) do -- then credit card companies can't insist that their business partners violate the law in those states. And the same applies to the cities and counties that have similar ordinances, which are sometimes broader than state law provides.