Before-and-after photos show how dramatically presidents have aged while in office

As President Barack Obama reflects on his time in office, he has a way of acknowledging his waning time in the position: A joke about his graying hair.

"Right now, we are waging war under authorities provided by Congress over 15 years ago - 15 years ago," he said during a speech reflecting on his administration's counterterrorism strategy.

"I had no gray hair 15 years ago."

Indeed, despite his close-cut hairstyle, it's impossible not to notice the trademark presidential graying, as the president's short black hair has become more of a salt-and-pepper color.

And though some dermatologists maintain that Obama is the latest victim of the expedited presidential aging process, appearing to age faster due to the stress of the office, others say that it's more attributable to natural aging than stress.

Other studies, including a comprehensive analysis of elections dating back to the 1700s, have found that heading a nation can take years off a leader's life. That analysis, from the Harvard Medical School, found that elected heads of government, on average, have lives almost three years shorter than the candidates they defeat.

Here's how the country's past presidents have looked near the beginnings and ends of their respective terms.