Wash. Rev. Code § 48.20.420
Any disability insurance contract providing health care services, delivered or issued for delivery in this state more than one hundred twenty days after August 11, 1969, which provides that coverage of a dependent child shall terminate upon attainment of the limiting age for dependent children specified in the contract, shall also provide in substance 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 (1) incapable of self-sustaining employment by reason of developmental or physical disability and (2) chiefly dependent upon the subscriber for support and maintenance, provided proof of such incapacity and dependency is furnished to the insurer by the subscriber 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.
RCW 48.20.420