So reducing lead exposure immediately changes your brain to do less crime?

No, there's an offset of about 18 years, if I remember correctly?

I see, so since a large majority of crime is done by young people, peaking between 15-25, they are basically comparing a whole new generation of kids who didn't have developmental brain issues vs their elders.

Were the older people who grew up with lead exposure also experiencing higher rates of impulsive crime in the late >1990s relative to the new and prior generations? That would help eliminate the major differences in economics/culture/politics of their upbringing (for ex: mass flight of families moving to the suburbs to raise their young kids after the 1970s crime wave scared them away).

That's an interesting question, and I don't know the answer.

Kids that grew up huffing leaded exhaust are more bad decisions inclined than they would otherwise be. It's not just crime. The most heavily leaded cohort in the US is also known for drunkly crashing their muscle cars and wasting their youth smoking pot in a commune.

Bad decisions like these get less common with age, partly because of consequences (jail, death, etc), partly because getting up to no good requires free time, ambition and freedom, all of which are in shorter supply with age and the resultant responsibilities competing for every individual's supply of these resources.

So if the replacement cohort of people who are coming into prime crime age decline to participate at the same rates the crime rate goes down.