All of these behavioral changes have their basis in anatomy. As the brain ages, it starts to look different. It loses about 2 percent of its weight and volume every decade, starting around age 20. Certain areas of the cortex get smaller and thinner, and its grooves and ridges become more pronounced. The spaces between the gyri—the wormy coils you might call brains—often get wider as well. (Gaps are especially pronounced in Alzheimer's patients.) But when it comes to cognition—memory, attentiveness, decision-making—it's not the stuff that matters so much as the stuff that connects the stuff, called "white matter." White matter is largely made up of conductive nerve strands sheathed in myelin that carry information from one area of the brain to another. In general, the more intact your white matter, the smoother your cognitive function. The quantity of myelin is thought to peak around age 50, after which white matter begins to disappear. (The opposite problem, white matter hyperintensity, is associated with late-life depression.)
The brain's chemistry also changes with age. Levels of dopamine, a neurotransmitter associated with sensations of happiness, generally decline. (Some schizophrenics, who experience abnormally high dopamine levels, recover suddenly when they reach a certain age.) Same with hormones like testosterone; on average, men in their 80s have testosterone levels 40 percent lower than men in their 20s. (Forget McCain, just ask Bob Dole.) The behavioral effects of lowered testosterone aren't known, but it's correlated with weaker cognition. But as recent history has shown, a president with low testosterone isn't necessarily a bad thing.
Emotional changes are harder to quantify, but on average, emotional stability increases over time—a so-called mellowing effect. Older people tend to concentrate on positive events over negative events. One study found that older adults shown positive images (like a bowl of ice cream) and negative images (like a dead animal) were 30 percent less reactive to the negative images than younger adults. Some attribute this response to end-of-life perspective—perhaps death doesn't seem quite so negative. Others point to a slowing of the limbic system, which regulates emotion.
Many studies now emphasize the cognitive benefits of aging. Researchers describe a greater appreciation for ambiguity among older people—some might even call it "wisdom." While the elderly are easily distracted, that inability to filter information efficiently often means they can take in more information. Likewise, while we know younger brains are speedier, there's little evidence that healthy older brains can't perform the same tasks, albeit more slowly. Roberto Cabeza of Duke University has conducted studies showing older adults often perform just as well as their younger counterparts because of what he calls "compensation." If one area of the brain is weaker, the theory goes, other areas will pick up the slack. For example, while younger people use the right side of their prefrontal cortex for high-level cognitive processing, some elderly people use the left side as well, resulting in equal performance.
How much of this describes McCain's brain? Who knows. It's impossible to apply general trends to an individual case, since so much depends on lifestyle (exercise, diet, smoking), genetics (there's a reason McCain trots out his 96-year-old mom), and environmental conditions (being educated helps; being tortured doesn't). Plus, changes in brain structure are highly variable. Average cognitive ability declines precipitously over time, but variability increases: Plenty of 70-year-olds outperform plenty of 20-year-olds. (My 90-year-old grandmother is sharp enough to be president 10 times over.) And don't forget all the ancient CEOs out there. Companies regularly hire septuagenarians for their wisdom, contacts, and job-specific skills. A recent survey of senior professionals found that 53 percent say they would hire a qualified 72-year-old CEO. And rightly so: Some jobs simply demand experience, even if it means having a leader who can't do long division in his head.
Whether the presidency is that sort of gig is for voters to decide.
Thanks to Suzanne Corkin of the Massachusetts Institute of Technology, Jeri Janowsky of Oregon Health and Science University, and Jennifer Manly of Columbia University.