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.