10 U.S.C. § 12108

Each reserve enlisted member of the Army, Navy, Air Force, or Marine Corps who is in an active status and has reached the maximum years of service or age prescribed by the Secretary concerned shall— (1) be transferred to the Retired Reserve if the member is qualified for such transfer and does not request (in accordance with regulations prescribed by the Secretary concerned) not to be transferred to the Retired Reserve; or (2) be discharged if the member is not qualified for transfer to the Retired Reserve or has requested (in accordance with regulations prescribed by the Secretary concerned) not to be so transferred.