Except as provided in subsection (c) of this section, a Comptroller General serving a complete term as Comptroller General or who retires under section 703(e)(1) of this title is entitled to receive an annuity for life equal to the pay the Comptroller General is receiving on completion of the term or at the time of retirement. An annuity of a Comptroller General who completes a term before becoming 65 years of age is reduced by .25 percent for each complete month the Comptroller General is under 65 years of age.