In the case of—

(A) an individual entitled to old-age insurance benefits (other than an individual referred to in subparagraph (B)), or

(B) an individual entitled to disability insurance benefits, or an individual entitled to old-age insurance benefits who was entitled to disability insurance benefits for the month preceding the first month for which he was entitled to old-age insurance benefits,

a child of such individual adopted after such individual became entitled to such old-age or disability insurance benefits shall be deemed not to meet the requirements of clause (i) or (iii) of paragraph (1)(C) unless such child—

(C) is the natural child or stepchild of such individual (including such a child who was legally adopted by such individual), or

(D) (i) was legally adopted by such individual in an adoption decreed by a court of competent jurisdiction within the United States, and (ii) in the case of a child who attained the age of 18 prior to the commencement of proceedings for adoption, the child was living with or receiving at least one-half of the child’s support from such individual for the year immediately preceding the month in which the adoption is decreed.