10 U.S.C. § 12102

(a) To become an enlisted member of a reserve component a person must be enlisted as a Reserve of an armed force and subscribe to the oath prescribed by section 502 of this title , or be transferred to that component according to law. In addition, to become an enlisted member of the Army National Guard of the United States or the Air National Guard of the United States, he must meet the requirements of section 12107 of this title . (b) Except as otherwise provided by law, the Secretary concerned shall prescribe physical, mental, moral, professional, and age qualifications for the enlistment of persons as Reserves of the armed forces under his jurisdiction. However, no person may be enlisted as a Reserve unless— (1) 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 (2) he has previously served in the armed forces or in the National Security Training Corps. (c) 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 enlisted as a Reserve of any armed force under the jurisdiction of that Secretary.