(a) In determining age, the agency must use the common law method (under which an age is reached the day before the anniversary of birth) or the popular usage method (under which a specific age is reached on the anniversary of birth), whichever is used under the corresponding State plan for OAA, AFDC, AB, APTD, or AABD.

(b) The agency may use an arbitrary date, such as July 1, for determining an individual's age if the year, but not the month, of his birth is known.