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.