By now we all know that vegetarians tend to have more education, and they never tire of reminding everyone else. However, this is a mere association, and so is pretty meaningless in trying to understand cause and effect. In particular, choice of diet is a marker of ethnic groups in the broad sense -- these people identify themselves in part by eating this diet, those others by eating that other stuff. All sorts of silly fads catch on among smart people, but it doesn't make them smarter. Does wearing a tie boost your brainpower?
But what about before veganism, organic food, and yoga became mainstream among the educated? Blogger Audacious Epigone looked at data from the General Social Survey, probably the largest and most representative survey of its kind, and found that while vegans were more educated, they weren't brighter than omnivores. (Vocabulary tests are highly reliable intelligence tests.) The survey question about eating meat vs. not eating meat was only asked in 1993 and 1994, so this could be a reflection of its lack of adoption by smarties at that point -- remember, this is nearly a decade or so before Whole Foods became the next Starbucks. (Both of which are great stores, of course.)
I decided to follow up and look at how IQ and diet were related across different age groups. We might expect diet to make a difference at any given age, but might diet choice cause us to cognitively age faster or slower? One weakness here is that the GSS only surveys people who are 18 or older, and it seems like the biggest effect of diet on the brain's horsepower would be at younger ages when it's still developing. With that said, here is how IQ and diet were related across age groups in 1993 - '94:
I've condensed all people who don't eat meat into one group, even though the survey measures three degrees of how frequently you abstain from meat, in order to keep sample sizes big. For the same reason, I've condensed people into 10-year age groups, with the center age ending in a 3. The two graphs show two different ways of measuring smarts -- by their average and by what percent are above the overall average (median to be exact). Each group has at least about 40 people, although the last age group is just about at this value for the vegetarians, so the 70-somethings may not have looked exactly like the data suggest.
In any case, there doesn't seem to be a strong change across the lifespan, since the gap between omnivores and vegetarians is fairly flat from the 20s through the 50s, an unclear change in the 60s, and again a not too reliable finding for the 70s. The aging pattern seems similar. However, at each age, omnivores are smarter than vegetarians by either measure. This means that either the damage was already done to vegetarians early in life, if they had mostly been vegetarian since childhood -- or that, if people face the "meat or no meat?" choice during their dopey-minded college years, brighter people were less likely to abandon animal products, while the dimmer found it more appealing. Who knows why -- it could be just as arbitrary of a choice of ethnic marker as any other, such as baggy vs. skinny jeans.
This point generalizes: if some group that does well in some respect happens to follow some diet, it may well be more of a group membership badge than a cause. You can find genetic freaks who will be ripped and athletic no matter what garbage they consume, so they can quickly move from one fashion to the next without suffering much. Hence the high turnover of nutrition and exercise advice given to people who are already pretty healthy and active, but who want to squeeze out that extra bit of energy. Only controlled studies, including long-term experiments like Darwinian natural selection, can tell us what happens to normal people, as far as cause and effect goes.