10 U.S.C. § 12102 — Reserve components: qualifications

(a) To become an enlisted member^* of a reserve component, a person must be enlisted as a Reserve of an armed force and take the oath required by section 502 of this title, or be transferred to that component according to law. To become an enlisted member of the Army National Guard of the United States^* or the Air National Guard of the United States^*, that person must also meet the requirements of section 12107 of this title.

(b) Except as otherwise provided by law, the Secretary concerned^* sets the physical, mental, moral, professional, and age qualifications for enlisting Reserves of the armed forces^* under that Secretary's jurisdiction. But no person may be enlisted as a Reserve unless:
(1) they are a citizen of the United States, or have 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) they have 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 performing the duties they may be assigned, may still be enlisted as a Reserve of any armed force under that Secretary's jurisdiction.