Sorry, can’t have it both ways.
Either it is age discrimination because an 18 yr old can legally purchase a fire arm and public accomodation forces busness to sell to everyone leaglly able to make the purchase.
-or-
You allow a christian baker to refuse to sell a cake to a homo.
Personally, I believe the first amendment freedom of assembly allows both the buyer and the seller, who both have voluntarily agreed to an exchange to go through. Should either party NOT want to participate, for any reason, they should be allowed to not conduct business.
But the left does not get to have it both ways.