42 CFR 435.522 - Determination of age.
(a) Except as specified in paragraphs (b) and (c) of this section, in determining age, the agency must use the common-law method (under which an age reached the day before the anniversary of birth).
(b) For families and children, the agency must use the popular usage method (under which an age is reached on the anniversary of birth), if this method is used under the State's AFDC plan.
(c) For aged, blind, or disabled individuals, the agency must use the popular usage method, if the plan provides under § 435.121, § 435.230, or § 435.330, for coverage of aged, blind, or disabled individuals who meet more restrictive eligibility requirements than those under SSI.
(d) 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.