New York City Police Commissioner William Bratton said Thursday he won't work beyond age 75, raising speculation he would not serve through Mayor Bill de Blasio's second term -- if there is one.

"I will not be commissioner for 61/2 years," Bratton said at a City & State-hosted event when asked if he'd still be on the job at the end of de Blasio's potential second term. "I'd be 70-some-odd, 75 years old by that time."

Bratton turns 68 in October. He won't turn 75 until 2022, a year after de Blasio would leave office if re-elected in 2017.

The mayor has 21/2 years left on his first term and has said he will run for a second four-year term.

In 2002 Bratton was appointed police chief of Los Angeles for two five-year terms but left in 2009.

The mayor said Thursday Bratton was not too old, adding that Pope Francis is in his 70s and still having a huge impact on global decisions.

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"I think it's fair to say that our concept of age needs to change," de Blasio said. "What the commissioner chooses to do with the future obviously is his choice, but I have immense respect for Bill Bratton . . . I think he's doing a great job and he should do it as long as he feels it's right for him and his family."

Regarding Bratton's remarks about his retirement age, police historian and author Thomas Reppetto said a police commissioner "is like a baseball manager. The day he is hired is the day he should be prepared to be fired. . . . I wouldn't put a lot of significance to this."

Bratton and de Blasio appear to have a good working relationship and the commissioner has repeatedly said in recent weeks that he has been happy with the mayor's latest budget giving the department hundreds of new officers.

Michael Paladino, head of the Detective Endowment Association, said he believed that while a de Blasio second term is not a sure thing, Bratton has the right to rest a bit on his laurels and what he has done with the NYPD since taking over in 2014.

"Bratton is entitled to take a deep breath and look back and enjoy his successes," Paladino said.

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With Emily Ngo

and Alison Fox