S.D. Codified Laws § 58-18-31
A group or blanket health insurance policy, which is delivered or issued for delivery in this state and which provides that coverage of a dependent child shall terminate upon attainment of the limiting age for dependent children specified in the policy, shall also provide that attainment of such limiting age shall not operate to terminate the coverage of such child while the child is and continues to be both (a) incapable of self-sustaining employment by reason of intellectual disability or physical disability and (b) chiefly dependent upon the policyholder for support and maintenance, provided proof of such incapacity and dependency is furnished to the insurer by the policyholder within thirty-one days of the child's attainment of the limiting age and subsequently as may be required by the insurer but not more frequently than annually after the two-year period following the child's attainment of the limiting age.
SDCL 58-18-31