32 USC 324 - Discharge of officers; termination of appointment

(a) An officer of the National Guard shall be discharged when
(1) he becomes 64 years of age; or
(2) his Federal recognition is withdrawn.

The official who would be authorized to appoint him shall give him a discharge certificate.

(b) Subject to subsection (a), the appointment of an officer of the National Guard may be terminated or vacated as provided by the laws of the State of whose National Guard he is a member, or by the laws of the Commonwealth of Puerto Rico, or the District of Columbia, Guam, or the Virgin Islands, whose[1] National Guard he is a member.
[1] So in original. Probably should be “of whose”.