32 U.S.C. § 313

(a) To be eligible for original enlistment in the National Guard, a person must be at least 17 years of age and under 45, or under 64 years of age and a former member of the Regular Army, Regular Navy, Regular Air Force, or Regular Marine Corps. To be eligible for reenlistment, a person must be under 64 years of age. (b) To be eligible for appointment as an officer of the National Guard, a person must— (1) be a citizen of the United States; and (2) be at least 18 years of age and under 64.