20 year. Try 70 years for light aircraft. Most Lycomings and Continental engines have remained basically unchanged for 70 years. They did introduce fuel injection at some some stage, but the pollution is basically the same as planes are flown using lean or rich of peak egt.But, if CARB went after airplanes and cruise ships they could make a real difference as they are the real polluters. Cars get new, cleaner engines every year. Planes use 20 year old tech in many cases. Its an unfortunate situation.
Also they use leaded fuel.
Having said that, There are a darn side more cars on the road, than light aircraft.
So probably why they are overlooked.
So you drive to the airport in your Prius, then go pollute the sky in you’re Beechcraft Baron haha.