It's not clear. While psychological stress can lead to DNA damage associated with aging, it's not clear whether this damage manifests itself visibly. But that hasn't stopped some news outlets from heralding a link between stress and graying hair. When a 2011 study showed a mechanism through which stress could cause DNA damage, articles in the Daily Mail, Yahoo, and elsewhere touted the study as proof that stress can cause visible aging. "Stress Really Does Make Your Hair Go Gray, Scientists Find," proclaimed the headline in the Telegraph.
However, the study had little or nothing to do with gray hair, with the words gray and hair never even appearing. Furthermore, the study used adrenaline, not stress, and it was conducted on mice, not people.
As William Saletan pointed out in Slate in 2009, many other studies have found no relationship between early graying and aging. A Danish study of 20,000 men and women could find no relationship between deaths from heart disease and outward signs of aging, such as balding, wrinkly skin, and gray hair. Instead, most graying seems to be determined by genetics. If presidents tend to go gray in office, it may simply be because most normal graying happens during the same years in which presidents serve.
"Overnight" graying from stress is extremely rare, if it exists at all. Though traumatized fictional characters have had their hair suddenly turn white since at least the days of Shakespeare, the phenomenon remains unproven.
Photo selection can also exaggerate the appearance of aging. While Obama's sprinkles of gray started making headlines just 44 days into his presidency, a Huffington Post article this week actually complimented the president's "amazing glow." A CNN commentator agreed, remarking that he "looks five years younger."