I dunno. I just thought it was pretty well known that licensed federal arms dealers are one of the few blanket exceptions to the Civil Rights Act, where they can refuse to sell a gun to anybody for pretty much any reason. Even if it's questionable, much like cops and the "I feared for my life" Get Out of Jail Free card, gun dealers have the "I felt he was buying the gun for someone else" card. But that one guy, in Florida I think it was, maybe after the Pulse shooting, put up a sign that said "No Muslims Allowed" or something like that. He was challenged in court, and the case was dismissed (IIRC, though, it was due to lack of standing of those who brought the lawsuit).
Dick's and maybe Walmart might be challenged in court over this, but it won't be on Civil RIghts grounds. There are a handful of states (New Jersey is one that I know of) that explicitly prohibit discrimination of goods and services based on age of anyone over 18 (most other states it's at 40 years old, or older). But I don't see such a lawsuit being successful. Dick's will either just pull all long guns from the stores in those states, or everyone under 21 will be "buying the gun for someone else.