Except as provided in this section and in section 504,¹ whenever any person under eighteen years of age admitted, committed or otherwise receiving any service or benefit under this act shall be unable to discharge the obligation imposed upon him by section 501,² such liability is hereby imposed upon any person owing a legal duty to support the person admitted, committed or otherwise receiving services or benefits under this act. Upon the mentally disabled person attaining the age of eighteen, or any mentally disabled person over eighteen years of age on the effective date of this act, the liability under the act of the persons owing a legal duty to support him shall cease: Provided, however, That spouses shall remain liable for each other regardless of age except for periods of continuous inpatient care in excess of one hundred and twenty days. Continuous inpatient care for the purposes of this section shall be any in-hospital stay not interrupted by more than one hundred and twenty days. Nothing in this section 502 shall relieve any private, nonprofit or governmental health insurer for liability to pay for such care under any contract of insurance or group insurance plan.