It a few states it is. It is why in New York and Michigan car rental agencies have to allow 18 year olds to lease a car. Those states specifically have laws that stop age discrimination in those categories. Other states do not and the car rental agencies are allowed to set their own age limits. Usually 20-25. Their polices have to follow local, state, and federal laws.
Oregon doesn't clearly define selling of products in their public accommodations law which means age discrimination there for fire arm sales likely won't be won. The copy cat case filed in Michigan however will likely be won because Michigan specifically mentions sales as a part where age can be used as a reason to discriminate.