"I mean there's no reason for me not to release my medical records," the 76-year-old said.

Near the beginning of Thursday night's debate, former housing and urban development secretary Julian Castro challenged that Biden was misstating something he'd said two minutes prior about his healthcare plan.

"Are you forgetting already what you said just two minutes ago? I mean, I can't believe that you said two minutes ago that they had to buy in and now you're saying that they don't have to buy in. You're forgetting that," Castro said.

The accusation that Biden's memory had faltered was interpreted by many viewers as a direct hit at Biden's age. Castro denies this, but it wouldn't be the first time younger Democrats in the crowded field have taken a swipe at Biden's age.

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Asked how he can prove to rivals that he can get the ball across the finish line, Biden said, "I carry the ball over the finish line."

Biden's 46-year political career is older than that of several of the candidates on the debate stage, including Castro. Biden's campaign says it's his breadth of experience that makes him the best equipped to take on President Donald Trump next year. Critics say Democrats need a fresh perspective, though the second highest polling candidates, Senators Elizabeth Warren and Bernie Sanders are also in their seventies, 70 and 78 respectively. Trump is 73.

Critics slammed Castro, who turns 45 on Monday, for what they saw as ageist attacks, but Biden said on Friday that shots at his age were fair game.

The Washington Post