CNN's Don Lemon has issued an apology hours after he insinuated that Republican presidential candidate Nikki Haley, who is 51 years old, is past her prime.

"This whole talk about age makes me uncomfortable," Lemon began. "Nikki Haley isn't in her prime, sorry. When a woman is considered to be in her prime in her 20s, and 30s and maybe 40s."

WATCH: DON LEMON SLAMS NIKKI HALEY AND SAYS SHE 'ISN'T IN HER PRIME'

Lemon, 56, then was quick to say it was "not according to" him and told the audience to "Google it."

When you do a web search as he advised the audience to do, the answer that pops up is, "A woman's prime is referring to an age range where a woman feels physically and sexually at her peek. Often, it's in the late 30s to early 40s but can extend far into the 50s."

Lemon's comments set off a firestorm on social media, including a response from Haley.

"Liberals can't stand the idea of having competency tests for older politicians to make sure they can do the job. BTW it's always the liberals who are the most sexist," she tweeted.

Now, Lemon has also taken to Twitter to apologize.

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"The reference I made to a woman’s “prime” this morning was inartful and irrelevant, as colleagues and loved ones have pointed out, and I regret it. A woman’s age doesn't define her either personally or professionally. I have countless women in my life who prove that every day," he wrote.

Lemon's show, CNN This Morning, has been struggling to gain an audience, drawing the lowest ratings for the network in a decade.