Your IQ at the age of eleven predicts your brain anatomy sixty years later, according to a Canadian/Scottish team of neuroscientists: Childhood cognitive ability accounts for associations between cognitive ability and brain cortical thickness in old age. The authors of the new paper, Karama et al, made use of a unique long-term study of Scottish volunteers, all of whom had IQ tests back in 1947, when they were 11. In 2009, aged 73, those who were still alive and willing to participate had an MRI brain scan - a total of 588 people. Karama et al show that childhood IQ is correlated with the thickness of the brain's cerebral cortex in old age. What's more, IQ at age 70 was correlated with brain anatomy, but no more closely than the age 11 scores were - that is to say, cortical thickness in old age is correlated, not just with IQ in old age, but with IQ at any age. As you can see on this image, the relationship was seen across most regions of the brain:
In other words, people with higher IQs just tend to have a thicker cerebral cortex across the lifespan. The authors point out, however, that they didn't include anyone with diagnosed or suspected dementia. It's certainly true that some forms of dementia cause rapid cortical thinning, and that this is associated with cognitive decline. But dementia aside, it seems that the brain you have at 73 is a reflection of your lifelong IQ. There's nothing special about old age which drives the relationship. This result is pretty remarkable, although the correlations were small (coefficients of 0.1 to 0.3, depending upon the region.) Why this correlation exists is another question. One interpretation is that having a thicker cortex makes you more intelligent, and so the correlation between IQ and anatomy would have also been true at age 11. However it could be that the smarter people took better care of their brains over 60 years, leaving them with more grey matter, even though this wasn't what made them smart in the first place.
Karama, S., Bastin, M., Murray, C., Royle, N., Penke, L., Muñoz Maniega, S., Gow, A., Corley, J., Valdés Hernández, M., Lewis, J., Rousseau, M., Lepage, C., Fonov, V., Collins, D., Booth, T., Rioux, P., Sherif, T., Adalat, R., Starr, J., Evans, A., Wardlaw, J., & Deary, I. (2013). Childhood cognitive ability accounts for associations between cognitive ability and brain cortical thickness in old age Molecular Psychiatry DOI: 10.1038/mp.2013.64