I think the lead exposure = more violence connection is pretty well established. Leaded gasoline, while being the most studied connections out there, isn't even the only study in this regard.

For instance, there's this rather extensive study that shows higher homicide rates (among other things) in cities that used lead pipes for their water systems in the early 20th century.

http://scholar.harvard.edu/files/jfeigenbaum/files/feigenbau...

I do not see anything at the moment connecting lead exposure to dementia though.