Recent research does seem to support the hypothesis that there may be a delayed mental aging effect caused by, among other things, decades-old lead exposures. That would be expressed mainly now for US citizens over 45.
The human "body burden" of lead falls off in the years following a period of high exposure: by excretion. With that knowledge, seeing lead's toxic effects in populations exposed while growing up in the 1960's seems insensible. On the other hand, cigarette smoking while a teen has an effect on lifetime risk of cancer. So, there you go.
In either case, the research of Dr's. Schwartz and Hu frames an interesting view of the libertarian philosophy of environmental management, which generally hangs on the supposition that 'when the risk becomes apparent, the private sector will act to control exposures.' Somewhat problematic with an effects lag-time of a half-century.
"...work by Schwartz and Dr. Howard Hu of the University of Michigan suggests that the long-term effects of the high-lead era are still being felt.
In 2006, Schwartz and his colleagues published a study of about 1,000 Baltimore residents. They were ages 50 to 70, old enough to have absorbed plenty of lead before it disappeared from gasoline. They probably got their peak doses in the 1960s and 1970s, Schwartz says, mostly by inhaling air pollution from vehicle exhaust and from other sources in the environment.
The researchers estimated each person's lifetime dose by scanning their shinbones for lead. Then they gave each one a battery of mental-ability tests.
In brief, the scientists found that the higher the lifetime lead dose, the poorer the performance across a wide variety of mental functions, such as verbal and visual memory and language ability. From low to high dose, the difference in mental functioning was about the equivalent of aging by two to six years.