20 CFR 404.2(c)(4)
In 1955, G, a girl born December 1, 1944, became entitled under section 202(d) of the Social Security Act to child's insurance benefits on the earnings record of her deceased father, a full insured worker.
Under the provisions of section 202(d), entitlement to child's insurance benefits (after being established) continues until termination is required by the occurrence of certain specified events. Among the events which can require termination is the child's attainment of age 18: his entitlement ends with the month before the month in which such attainment occurred unless the child is under a disability (as defined in section 223(c)(2), which began before he attained age 18.
In G's case, no event requiring termination of her entitlement occurred until she attained age 18. At no time prior to the attainment of age 18 was she under a disability. The question presented is whether she attained age 18 on December 1, 1962, her 18th birthday (i.e., the 18th anniversary of her birth) or on the preceding date, November 30, 1962. If she attained age 18 in December 1962, under section 202(d) her entitlement ends with the month of November 1962; if she attained age 18 in November 1962, her entitlement ends with October 1962.
Section 202(d) is only one of a number of provisions in the Act under which benefit rights are affected by attainment of a specified age. It is commonly necessary under other provisions in section 202 to decide in what month a claimant "has attained age 62," as required for entitlement to old-age, widow's, husband's, widower's, and parent's insurance benefits, and (subject to certain exceptions) wife's insurance benefits. Similarly, under section 203, benefits may be paid to a beneficiary beginning with the month he attains age 72 even though his work and earnings in such months would preclude payments if he had not attained age 72.
A person attains a given age on the day before his corresponding birthday, i.e., the anniversary of his birth corresponding to that age. (§ 404.2(c)(4) of Regulations No. 4.) Accordingly, for purposes of benefits under the Act, a person born on the first day of a month attains any specified age in the month before the month in which occurs the corresponding birthdate. For example, a person born on May 1 attains age 62 on the day before his 62nd birthday, that is, April 30 of that year; and may become entitled to old-age insurance benefits beginning with that April. Similarly, he attains age 72 in April immediately preceding his 72nd birthdate, and may be paid benefits for that April and subsequent months regardless of the amount of his work and earnings. Under the same rule, he attains age 18 on April 30, immediately preceding his 18th birthday; and, where such attainment requires termination of entitlement to child's insurance benefits, entitlement ends with the preceding month.
Since G's 18th birthday was December 1, 1962, it is held, that she attained age 18 on the preceding day, November 30, 1962. Therefore, the last month for which she was entitled to child's insurance benefits is October 1962, the month preceding the month in which she attained age 18.
 This rule, which appears in Blackstone's Commentaries (Vol. I, p. 463) and in Kent's Commentaries (Vol. 2, p. 233) has been applied consistently by the courts in holding that an individual attains majority on the day before his 21st birthday. United States v. Wright, 197 Fed. 297 (1912). Nor is the rule confined to cases involving the attainment of majority; see, for example, Frost v. State, 45 So. 203 (1907) involving attainment of age 45.