Sen. Bernie Sanders, in his first major public appearance since suffering a heart attack this month, raised the issue as he tried to insert himself into a debate question about the opioid epidemic.
"I'm healthy, I'm feeling great," the Vermont independent said, leading moderator Erin Burnett to ask the 78-year-old how he would reassure the American people after his health scare.
"Well, let me invite you all to a major rally we're having in Queens, New York," Sanders said, before promising a special guest and touting his website. "We are going to be mounting a vigorous campaign all over this country that ... I think can reassure the American people."
Sanders then grew somber as he thanked supporters and his fellow candidates for their well-wishes.
"I'm so happy to be back here with you this evening," Sanders said.
Burnett turned to the other septuagenarians onstage to ask about their age in relation to their electability.
Former Vice President Joe Biden, 76, and Massachusetts Sen. Elizabeth Warren, 70, said their ages would not affect their campaigns as they try to take down President Donald Trump, 73.
"One of the reasons I'm running is because of my age and experience _ with it comes wisdom," Biden said, adding that he would release his medical records before the first voting occurred in February.
Warren, who is frequently seen sprinting to the stage at her events, added that age was less important than providing an ambitious plan to attract the American electorate. But she also pointed to her vigor.
"I will outwork, out-organize and outlast anyone, and that includes Donald Trump, Mike Pence or whoever the Republicans get stuck with," Warren said.
