A special license is not required to operate an off-road vehicle (ORV) in Washington. However, wheeled all-terrain vehicles (WATVs) must have a metal tag. Anyone under the age of 16 must simply be supervised by a licensed driver over the age of 18. If you intend to drive your vehicle on a roadway, you must have a valid driver's license.
Check out these Des Moines, WA ATV/OHV resources: