In Class 1-D-D shall be placed any registrant who:
enlists or accepts an appointment before attaining the age of 26 years, in the Ready Reserve of any Reserve component of the Armed Forces, the Army National Guard, or the Air National Guard. Such registrant shall remain eligible for Class 1-D-D so long as he serves satisfactorily as a member of an organized unit of such Ready Reserve or National Guard, or satisfactorily performs such other Ready Reserve service as may be prescribed by the Secretary of Defense, or serves satisfactorily as a member of the Ready Reserve of another reserve component, the Army National Guard, or the Air National Guard, as the case may be; or
32 C.F.R. §1630.13