For purposes of this subchapter, the term “child” means an individual who is neither married nor (as determined by the Commissioner of Social Security) the head of a household, and who is (1) under the age of eighteen, or (2) under the age of twenty-two and (as determined by the Commissioner of Social Security) a student regularly attending a school, college, or university, or a course of vocational or technical training designed to prepare him for gainful employment.