Any certificate issued by a health care corporation that provides that coverage of a dependent of the subscriber terminates at a specified age shall not terminate with respect to an unmarried child who is incapable of self-sustaining employment by reason of developmental disability or physical disability, if the following conditions are met:

    (a) The child became incapable before 19 years of age and is chiefly dependent upon the subscriber for support and maintenance.

    (b) Before the child turns 19 years of age, or within 31 days after that, the subscriber has submitted proof of the dependent's incapacity to the corporation.