President Obama, shown at left in December 2008 and at right on Aug. 2, 2016. (Charles Dharapak/AP (left); Joshua Roberts/Reuters (right))

If there’s one thing the sharply divided nation can agree on after nearly eight years of President Obama, it is that things have changed on his watch.

Most notably, his appearance.

Even the president has noticed. Opening his speech at the Democratic National Convention last week, Obama praised his wife, Michelle, the 52-year-old first lady, who “somehow hasn’t aged a day.”

“I know the same cannot be said for me,” Obama continued with a chuckle. Twelve years after a wunderkind 42-year-old Illinois state senator burst onto the national scene with a youthful vigor and forward-looking exuberance, the president’s once-black hair has gone nearly fully gray. The creases around his mouth and under his eyes have grown deeper.

“My girls remind me all the time: ‘Wow, you’ve changed so much, daddy,’ ” he said. “And then they try to clean it up. Not bad, just more mature.”

On Thursday, the 44th president turned 55.

It was his final birthday in office, and came at a time when Obama is acknowledging that his own political shelf life is winding down. Though mid-50s is young for an outgoing commander-in-chief, and he will presumably enjoy a long, active post-presidency, Obama has grown increasingly wistful in his final year.

“It’s true, I was so young that first time in Boston,” he said last week, reflecting on the speech at the 2004 convention that helped launch his political rise.

At times on the world stage, Obama has taken on the tone of the wisened dad, or at least big brother, dispensing advice to a new generation of younger leaders.

“I had no gray hair when I was in your shoes seven years ago,” Obama told Canada’s boyish Prime Minister Justin Trudeau, then 43, during their first bilateral meeting last November. “You need to start dyeing it soon, because it gets too late.”

“So young — and yet so cynical,” Trudeau interjected with a laugh. But the prime minister then revealed what Obama told him during a congratulatory phone call after Trudeau was elected.

“What Barack pointed out to me about how fast his daughters grew in the seven years since that first night of his electoral victory really struck home for me,” Trudeau said.

It may have been a day early, but members of the Young African Leaders Summit stood up to sing "Happy Birthday" to President Obama. Obama thanked the crowd joking, "Let me first of all just say that I am a little disappointed with the lack of enthusiasm." (Reuters)

In June, Obama declined an invitation to speak at older daughter Malia’s graduation from Sidwell Friends School, for fear that he might tear up.

It has become something of a habit for the public to compare before and after shots of two-term presidents and to fret about how much the weight of responsibility for the nation’s security and prosperity takes its toll on each occupant of the White House. That pastime has become accentuated in the social media age; Twitter users bandied about mock-ups of the 2004 Obama and the 2016 Obama during his convention speech last week.

For Obama, the gray hair is so obvious that it has become part of his stump speech, a way to disarm his audience by poking fun at himself. But by other measures, he has remained fit and vibrant.

Though he has hung up his basketball sneakers, Obama’s morning gym workouts and frequent golf course outings, along with his famous personal discipline at the snack table, have left him not just leaner than he was two years ago — but also, in­cred­ibly, taller.

Obama checked in at 175 pounds and 6 feet 1 1 / 2 inches on his most recent personal physical — five pounds less and a half-inch taller than in 2014, according to a medical report released by the White House in March.

Once a smoker, the president remains tobacco free and only drinks alcohol in moderation; his bad cholesterol is low and his good cholesterol is high, the exam found.

“All clinical data indicates that the President is currently very healthy and that he will remain so for the duration of his Presidency,” his doctor, Ronny Jackson, concluded.

Obama has used past birthdays to have a date night with the first lady, raise money for his 2012 campaign and shoot hoops with professional basketball stars.

This year, though, the president has surrounded himself with friends from the good old days. Obama was joined on the golf course at Camp David last weekend by Chicago neighbor Martin Nesbitt, high-school pals Mike Ramos and Bobby Titcomb, and college friends Laurent Delanney and Hasan Chandoo.

On Thursday, the president held a private briefing with his national security team at the Pentagon, which was followed by a news conference. He will depart on his annual two-week vacation to Martha’s Vineyard on Saturday. White House officials declined to say whether he was planning any birthday celebration.

Even if Obama is suddenly feeling more in touch with his mortality, he can take some solace that he will be replaced by someone a lot older — Hillary Clinton is 68 and Donald Trump is 70. But that seems of little comfort to a president who took office at 47 years old, the fifth-youngest to assume the White House.

At a town hall-style forum with young African leaders in Washington on Wednesday, Obama took his first question from a man who felt a kinship with the president, whose father was born in Kenya.

“In this room, we’re all brothers,” the man told Obama.

The president shrugged.

“I have to say that at this point I’m probably an uncle,” he replied. “I wish I could say I was a brother or a cousin, but now I’ve got some gray hair, so you’ve got to call me uncle.”