No age listed at the state level. But pretty clearly says that it can be listed at the city or county level:
No agency filings affecting this section since 2003
Water ski observer.
Qualifications. These rules are to establish minimum requirements for the observer required when water skiing on the waters of the state.
(1) The observer shall be physically capable and mentally competent of performing the following requirements:
(a) Continuously observe the skier or skiers;
(b) Inform the operator of the vessel towing the skier or skiers when they are no longer being towed;
(c) Raise the skier down flag as required when a person is in the water preparing to ski or returning to the ski vessel.
(2) Nothing in this section shall preempt local government from exercising any power that it possesses under the laws or Constitution of the state of Washington to adopt more stringent regulations on the qualifications for water ski observer.