Lead levels in the U.S. declined quickly after the banning of lead in gasoline. The year 1976 represented a peak for lead, after which followed a steep decline. Children from 1979 onward grew up with a fraction of the exposure to lead that their older siblings would have had.
If the lead theory is correct, what we should see then is a decline starting about 1993 or so, as lead-free young males move into the 16-24 demographic associated with violent crime. (Source)