(1) “dependent child” means an unmarried dependent child (including a stepchild or adopted child) who is— (A) under 18 years of age; (B) incapable of self-support because of physical or mental disability; or (C) between 18 and 22 years of age and is a student regularly pursuing a full-time course of study or training in residence in a high school, trade school, technical or vocational institute, junior college, college, university, or comparable recognized educational institution. For the purposes of this subchapter, a child whose 22nd birthday occurs before July 1 or after August 31 of a calendar year, and while such child is regularly pursuing such a course of study or training, is deemed to have become 22 years of age on the first day of July after that birthday. A child who is a student is deemed not to have ceased to be a student during an interim period between school years if the interim period is not more than 5 months and if such child shows to the satisfaction of the General Counsel of the Government Accountability Office that such child has a bona fide intention of continuing in the same or a different school during the school semester (or other period into which the school year is divided) immediately after the interim period.
31 USC § 771(1)
For the purposes of this subchapter