From Reason:

Oregon is one of the states that bans retailers from discriminating based on age against customers age 18 and above. (Some states don’t have such rules, or have an age 21 cutoff.) The Oregon statute says it generally applies to any person who is “of age,” which appears to mean 18, the age of majority in Oregon, at least for those products that are legal to sell to 18-to-20-year-olds (as long guns are in Oregon). Indeed, the statute specifically mentions alcohol and marijuana sellers for special treatment, but makes no such special provision for gun sellers