A district may lease all lands, wharves, docks and real and personal property owned and controlled by it, for such purposes and upon such terms as the port commission deems proper: PROVIDED, That no lease shall be for a period longer than fifty years with option for extensions for up to an additional thirty years, except where the property involved is or is to be devoted to airport purposes the port commission may lease said property for such period as may equal the estimated useful life of such work or facilities, but not to exceed seventy-five years: PROVIDED FURTHER, That where the property is held by the district under lease from the United States government or the state of Washington, or any agency or department thereof, the port commission may sublease said property, with option for extensions, up to the total term and extensions thereof permitted by such lease, but in any event not to exceed ninety years.