"Right now, we are waging war under authorities provided by Congress over 15 years ago 15 years ago," he said in December 2016.
"I had no gray hair 15 years ago."
Indeed, despite his close-cut hairstyle, it was impossible not to notice the trademark presidential graying, as the president's short black hair became more of a salt-and-pepper color.
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 those of the candidates they defeat.
Here's how past US presidents have looked near the beginnings and ends of their respective terms:
President Donald Trump was 70 when he took office, the oldest in history.
Now 72, he doesnt look much different.
Obama looked youthful when he took the oath of office on Inauguration Day, January 20, 2009.
By his year-end 2016 news conference at the White House, he was weathered.
Heres George W. Bush making a phone call shortly after the 2000 election.
And heres Bush fielding questions during his final White House press briefing, on January 12, 2009.
Hillary Clinton and Bill Clinton looked energetic at a dinner in 1993 several days before his first inauguration.
Heres Clinton giving a brief speech toward the end of his term in October 2000.
Appearing without his trademark glasses, then-Vice President George H.W. Bush answered a question at the second presidential debate in October 1988.
By June 1992, as Bush addressed a crowd of veterans during a ceremony at the Korean War Memorial, several months before losing the presidential election, he looked older.
Former President Ronald Reagan and first lady Nancy Reagan greeted fans lined up in Washington at his first inauguration in January 1981. Though he was 69, his movie star appearance held up.
When Reagan returned to Washington after his final trip as president to Camp David in January 1989, he looked quite different.
Though the photo is black-and-white, you can see Jimmy Carter emerging from a Georgia voting booth on Election Day in November 1976.
Heres Carter preparing for his farewell address to the nation in January 1981.
President Richard Nixon gave a press conference in the East Room of the White House several weeks after being sworn in in 1969.
Plagued by the Watergate scandal, a glassy-eyed Nixon delivered a final speech for White House staff and members of his Cabinet in 1974.
AP Photo/Charlie Harrity, File
President Lyndon Johnson proclaimed a day of mourning for deceased President John F. Kennedy shortly after being sworn in in 1963.
Johnson, who didnt visibly age too much in his five-year tenure, joins Nixon shortly after Nixon is elected president in November 1968.
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