That doesn't fly at all in a country that compels businesses to serve those they do not wish to under public accommodation laws that the courts say is constitutional.
If the law says something is legal for everyone 18+, how can Walmart anymore deny a sale to someone 18, 19 or 20 than they could deny people over 50 for being "too old"?
It may not fly in your mind, but as a business owner, I decide what goes on in my store, not you. If you don’t want to shop here, that is fine. You shop where you want. But, I make the rules here. End of story.