20 CFR 222.53: When a legally adopted child is dependent—child adopted after entitlement.

A child who is not the employee's natural child or stepchild, and who is adopted by the employee after the employee could become entitled to an old age or disability benefit under the Social Security Act (treating his or her railroad compensation as wages under that Act), is considered dependent on the employee during the employee's lifetime only if—

(a) The child had not attained age 18 when adoption proceedings were commenced, and the child's adoption was issued by a court of competent jurisdiction within the United States; or

(b) The child had attained age 18 before adoption proceedings were commenced, the child's adoption was issued by a court of competent jurisdiction within the United States, and the child was living with or receiving at least one-half of the child's support from the employee for the year immediately preceding the month in which the adoption was issued.