Child
Child means, except with reference to lump-sum payments, an unmarried child, under the age of 18 years, or such unmarried child regardless of age who because of physical or mental disability incurred before age 18 is incapable of self-support. In addition to the offspring of the participant, the term includes:
(1) An adopted child;
(2) A stepchild or recognized natural child who received more than one-half support from the participant; and
(3) A child who lived with and for whom a petition of adoption was filed by a participant, and who is adopted by the surviving spouse of the participant after the latter's death. “Child” also means an unmarried student under the age of 22 years. For this purpose, a child whose twenty-second birthday occurs before July 1 or after August 31 of a calendar year, and while a student, is deemed to have become 22 years of age on the first day of July after the birthday.