You have been downvoted, but you have a point. In the 1970s, you could buy both leaded and unleaded gas at service stations. New cars used unleaded gas, but old ones "required" leaded for various reasons that turned out to be mostly wrong (http://www.stevelinden.com/unleaded-gas-vs-regular-gas-in-cl...). Unleaded was required to work with the catalytic converters that were newly required in cars.

Regular leaded gas was cheaper. My neighbor had the catalytic converter in his car removed (it was illegal) so he could use the cheaper leaded gas.

So there would be an effect in which old cars imply more lead.

“It’s the model made before catalytic converters, so it’ll run good on regular gas. Whaddaya say, is it the new bluesmobile or what?”

— Elwood Blues, 1980, The Blues Brothers

Not sure why it was downvoted but maybe those people think it was because abortion boom during this time after ruling that legalized it in some states and hence, would be criminals never being borned that attributed to fall in crime rates.

Maybe it's even a combination of both.

Lead emissions come from the gasoline that's burned, not the cars burning it. Once leaded gasoline was eliminated, lead emissions from cars ceased.

New cars are required to be fitted with catalytic converters that reduce pollutants.

Those help burn stuff like hydrocarbons and carbon monoxide in the exhaust. It's unrelated to lead.

Not entirely unrelated. Exhaust catalysts are easily "poisoned," by lead among other things. Leaded gasoline would do it, so modern catalysts would never have been fitted to cars permitted to run leaded gasoline (very relevant during the time when both leaded and unleaded were available). Or put another way, the "unleaded only" signs you see on some older cars was not because the cars wouldn't run, but because running leaded gasoline would quickly degrade the emissions control equipment (oxygen sensors and catalysts primarily)