As used in this article “child” means a member’s child who, when the member dies, is both dependent on the member and unmarried, as well as any of the following:
(a) Under 18 years of age.
(b) Any age, totally disabled, and became totally disabled before reaching 18 years of age.
(c) Age 18 years or over, but under 22 years of age, and enrolled as a full-time student in an accredited school, as determined by the board.