Former New Jersey Governor Chris Christie compared President Donald Trump to a stubborn "72-year-old relative" who won't change his behavior.

"When people get older they become more and more convinced of the fact that what they’re doing is the right thing, and it becomes harder to convince them otherwise," he said.

He provided the comments as advice to Mick Mulvaney, the incoming White House chief of staff.

Trump ally and former New Jersey Governor Chris Christie compared the president to a stubborn elderly relative on ABC News Sunday, advising incoming chief of staff Mick Mulvaney to keep his expectations in check.

"I want to ask everybody who's out in the audience today if they have a 72-year-old relative whose behavior they're attempting to change," he said. "When people get older they become more and more convinced of the fact that what they’re doing is the right thing, and it becomes harder to convince them otherwise."

Trump is 72 years old and was the oldest person in history to be elected to a first term in office as president (Ronald Reagan was 73 when he began his second term). Christie made his comments as a way to illustrate the limits of the White House chief of staff role.

"Manage the things you can manage. And if Mick does that, I think he'll be successful. If he doesn't, it'll be hard," Christie said.

Read more: Donald Trump is the oldest president elected in US history

—ABC News Politics (@ABCPolitics) December 23, 2018

Mulvaney is set to take over as acting Chief of Staff in 2019, after John Kelly leaves the position. He's currently the director of the Office of Management and Budget in Trump's cabinet, and served as the head of the Consumer Financial Protection Bureau until earlier in December.

Christie himself was one contender for the role but pulled himself out of the running on December 14.

"It's an honor to have the President consider me as he looks to choose a new White House chief-of-staff," Christie said in a statement. "However, I've told the President that now is not the right time for me or my family to undertake this serious assignment. As a result, I have asked not to be considered for this post."