Oregon's anti-discrimination law encompasses many traits, including age. You cannot refuse service or sale to someone based on their age in the state, if the person is otherwise legally allowed to purchase a service or product. State and federal law says you you must be at least 18 to purchase a long gun, so the stores within the state have no choice but to follow the law. This is the same law that got that cake shop into a lot of trouble for refusing to sell a couple a gay wedding cake.
Relevant Oregon law, ORS 659A.400