10 U.S.C. § 14509

Each reserve officer of the Army, Navy, Air Force, or Marine Corps in a grade below brigadier general or rear admiral (lower half) who has not been recommended for promotion to the grade of brigadier general or rear admiral (lower half) and is not a member of the Retired Reserve shall, on the last day of the month in which that officer becomes 62 years of age, be separated in accordance with section 14515 of this title.