For the purposes of subsections (b)(1), (c)(1), (g)(1), (q)(5), and (q)(7) of this section and paragraphs (2), (3), and (4) of section 403(c) of this title , a child who is entitled to child’s insurance benefits under subsection (d) for any month, and who has attained the age of 16 but is not in such month under a disability (as defined in section 423(d) of this title ), shall be deemed not entitled to such benefits for such month, unless he was under such a disability in the third month before such month.