I'd say unleaded gasoline is not the most important factor. For Europe, I think it was drug use.
Also, European countries have some of the lowest crime rates in the world. If the young of the 2000s were under its influence, the effect is not large(not larger than 1-2 homicides per 100K people).
I believe that drug abuse during the 80s and 90s was the culprit of high crime rates in Europe. People addicted to drugs(heroin, crack, cocaine, etc.) committed lots of crimes. In the late 90s and early 2000s, Europe managed to lower drug abuse and then crimes dropped. For Mexico, there is likely another explanation. Like drug trafficking for instance.
Having said this, I think leaded gasoline has adverse effects on public health. I believe it can create mental problems on people.
However, not everyone exposed to leaded gasoline will develop those problems. And among those affected, not all of them will become criminals only because of lead poisoning. Therefore, I don't think that the majority of criminals during the 90s were so because of leaded gasoline. I think that other factors were more important, like drug abuse.