10 U.S.C. § 14512 — Separation at age 66: officers holding certain offices

(a) Army^* and Air Force.
(1) Unless retired, transferred to the Retired Reserve, or discharged at an earlier date, a reserve Army or Air Force officer^* described in paragraph (2) must be separated under section 14515 of this title on the last day of the month the officer turns 66.
(2) Paragraph (1) applies to a reserve officer of the Army or Air Force who is: (A) the Chief of the Army Reserve, Chief of the Air Force Reserve, Director of the Army National Guard^*, or Director of the Air National Guard^*; (B) an adjutant general; or (C), for a reserve officer of the Army, the commanding general of the troops of a state.

(b) Navy and Marine Corps.
(1) The Secretary of the Navy may defer, under section 14510 or 14511, the retirement of a reserve Navy officer above the grade^* of captain or a reserve Marine Corps officer above the grade of colonel, keeping the officer in active status^* until age 66. No more than 10 officers total may be deferred this way at any one time, split between the Navy Reserve and Marine Corps Reserve as the Secretary decides.
(2) The Secretary of Defense may separately defer the retirement of whoever serves as Chief of the Navy Reserve or Commander of the Marine Forces Reserve, but not past the first day of the month after that officer turns 66. This deferment does not count toward the 10-officer limit in paragraph (1).