to elect the annuity provided under clause (1) of
section 1434(a) of this title in place of the annuity provided under clause (3) of such section, if on the first day for which retired or retainer pay is granted the member had in effect a valid election under clause (3) of such section, and he does not have a child beneficiary who would be eligible for the annuity provided under clause (3) of such section. For this purpose, a child (other than a child who is incapable of supporting himself because of a mental defect or physical incapacity existing before his eighteenth birthday) who is at least eighteen, but under twenty-three years of age shall not be considered an eligible beneficiary; or