That's incorrect. It's correlation, not causation.

If lead were such a direct cause of violence, they should start diverging at the introduction of lead, not long after a notable decline in lead levels. (ok ok not diverge at first, but still have a predictable, visible connection throughout)

Children grow up constantly, and only one of the lines is not age-restricted. If it were to ever diverge while still being a major cause, it'd begin to diverge at the introduction of the stimulus... or rather, start to diverge when the supposed initiating cause was reduced.

As you say, lead is a lifetime problem. If it were to be a tent pole contributor to violence, violence wouldn't have reduced in lockstep basically ever.

Lead is a massive problem and is a toxin, but it surely did not itself contribute to increased violence. Increased stupidity? Absolutely.