10 U.S.C. § 12201: Reserve officers: qualifications for appointment

To become an officer of a reserve component a person must be appointed as a Reserve of an armed force in a grade corresponding to a grade authorized for the regular component of the armed force concerned and, except as provided in paragraph (2), subscribe to the oath prescribed by section 3331 of title 5 . In addition, to become an officer of the Army National Guard of the United States or the Air National Guard of the United States, he must first be appointed to, and be federally recognized in, the same grade in the Army National Guard or the Air National Guard, as the case may be. If an officer is transferred from the active-duty list of an armed force to a reserve active-status list of an armed force in accordance with regulations prescribed by the Secretary of Defense, the officer is not required to subscribe to the oath referred to in paragraph (1) in order to qualify for an appointment under that paragraph. he is a citizen of the United States or has been lawfully admitted to the United States for permanent residence under the Immigration and Nationality Act ( 8 U.S.C. 1101 et seq.); or he has previously served in the armed forces or in the National Security Training Corps. A person who is otherwise qualified, but who has a physical defect that the Secretary concerned determines will not interfere with the performance of the duties to which that person may be assigned, may be appointed as a Reserve of any armed force under the jurisdiction of that Secretary. In prescribing age qualifications under subsection (b) for the appointment of persons as Reserves of the armed forces under his jurisdiction, the Secretary concerned may not prescribe a maximum age qualification of less than 47 years of age for the initial appointment of a person as a Reserve to serve in a health profession specialty which has been designated by the Secretary concerned as a specialty critically needed in wartime.