(a) Officers below grade of rear admiral (lower half) Unless retired or separated earlier, each officer on the lineal list of the commissioned corps who is serving in a grade below the grade of rear admiral (lower half) shall be retired on the first day of the month following the month in which the officer becomes 62 years of age. (b) Flag officers Notwithstanding subsection (a) of this section, the President may defer the retirement of an officer serving in a position that carries a grade above captain for such period as the President considers advisable, but such a deferment may not extend beyond the first day of the month following the month in which the officer becomes 64 years of age.