In this chapter, "dependent" means:
(1) a spouse of a full-time employee or part-time employee;
(2) a child of a full-time or part-time employee if the child is younger than 26 years of age, including:
(A) an adopted child or child who is lawfully placed for adoption;
(B) a foster child, stepchild, or other child who is in a regular parent-child relationship; and
(C) a natural child;
(3) a full-time or part-time employee's natural child, adopted child, foster child, stepchild, or other child who is in a regular parent-child relationship and who lives with or has his or her care provided by the employee or the surviving spouse on a regular basis, regardless of the child's age, if the child has a mental disability or is physically incapacitated to an extent that the child is dependent on the employee or surviving spouse for care or support, as determined by the board of trustees; and
(4) notwithstanding any other provision of this code, any other dependent of a full-time or part-time employee specified by rules adopted by the board of trustees.