District of Columbia Code Division IV. Criminal Law and Procedure and Prisoners. § 22-4507. Certain sales of pistols prohibited.

No person shall within the District of Columbia sell any pistol to a person who he or she has reasonable cause to believe is not of sound mind, or is forbidden by § 22-4503 to possess a pistol [now “firearm”], or, except when the relation of parent and child or guardian and ward exists, is under the age of 21 years.