5 C.F.R. § 842.209

(a) A Member, except one separated by resignation or expulsion, is entitled to an annuity—

(1) After completing 25 years of service; or

(2) After becoming age 50 and completing 20 years of service.

(b) An annuity payable under paragraph (a) of this section commences on the day after separation from the service.