20 C.F.R. § 416.1856

Who is considered a child.

        We consider you to be a child if—

        (a)(1) You are under 18 years old; or

        (2) You are under 22 years old and you are a student regularly attending school or college or training that is designed to prepare you for a paying job;

        (b) You are not married; and

        (c) You are not the head of a household.

Official source