Julian Castro, 44, unloaded on Joe Biden, 76, about his age at the Democratic debate Thursday night, implying the former vice president was having memory problems in a not so subtle shot at concerns about Biden's advancing years.

'Are you forgetting what you said two minutes ago?,' Castro snapped at him during a fierce back-and-forth on healthcare as the audience gasped at the attack line.

'Are you forgetting already what you just said two minutes ago?,' he repeated as Biden looked shocked at the punches coming his way.

The image of one of the youngest candidates on the stage attacking one of the oldest comes as Biden, the frontrunner for the Democratic nomination, has faced increased questions about his health.

Biden was the main target of the night from the start of the debate when he got into a three-way tiff with liberal rivals Elizabeth Warren and Bernie Sanders on healthcare.

Julian Castro, 44, attacked Joe Biden, 76, about his age on the Democratic debate stage Thursday night

The former vice president attacked the cost of their proposed Medicare for All, sparking a 30 minute battle between the candidates on the health care issue.

Castro, who served as Secretary of Housing and Urban Development under President Obama, joined in the bashing of Biden.

When the moderator called on him, Castro first thanked the former president for his Obamacare plan and then pivoted to attack Biden for his healthcare plan, which is extension of Obamacare.

'We owe a debt of gratitude to President Barack Obama. Of course, I also worked for President Obama, Vice President Biden, and I know that the problem with your plan is that it leaves 10 million people uncovered,' he said.

'Now, on the last debate stage in Detroit, you said that wasn't true when Senator Harris brought that up. There was a fact check of that, they said that was true,' Castro added.

Castro, who is near the bottom of Democratic polls on the race, then when on to layout his healthcare plan and get in another shot at Biden.

'I want every single American family to have a strong medicare plan available. If they choose to hold onto strong, solid private health insurance, I believe they should be able to do.

'But the differences between what I support and what you support, Vice President Biden, is that you require them to opt in and I would not require them to opt in, they would automatically be enrolled.

'That's a big difference, because Barack Obama's vision was not to leave 10 million people uncovered. He wanted every single person in this country covered. My plan would do that. Your plan would not,' he said.

Biden defended his plan: 'They do not have to buy in. They do not have to buy in.'

And then Castro attacked him hard, implying he had forgotten what his plan involved in a not-so-subtle hint at the concerns among some Democrats about Biden's age.

'You just said that two minutes ago. You just two minutes ago they would have to buy in,' he said.

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

'You are automatically enrolled, ' Biden responded.

But Castro stayed on the attack.

'If you lose your job, for instance, his health care plan would not automatically enroll you. You would have to opt in. My health care plan would. That ice a big difference. I'm fulfilling the legacy of Barack Obama and you're not,' Castro said.

Aftermath: Julian Castro tried to speak to Biden at the end of the debate with the former vice-president, but he stared straight ahead as Andrew Yang looked round at Castro

Stare-down: Joe Biden turned to Julian Castro as the contenders mingled on stage after the debate but what he said to the younger man was unclear

'That will be a surprise to him,' Biden snapped back.

Earlier in the debate, when talking about his plan in comparison to Medicare for All, Biden said: 'If you want medicare, if you lose the job from your insurance company, from your employer, you automatically can buy into this. You don't have no pre-existing condition can stop you from buying in. You get covered. Period.'

Castro was referring to an attack line Harris threw at Biden in the second Democratic debate, which took place in Detroit in July.

She charged Biden's plan 'leaves out almost 10 million Americans.'

Fact checkers, after the debate, declared her attack line was correct.

Biden’s plan - which expands the Affordable Care Act by creating a government-backed public health insurance option and increasing Obamacare's federal subsidies – would insure more than an estimated 97 per cent of Americans.

That means out of the population of 327 million in the United States, roughly 10 million would be left without any health insurance.

Castro was repeating an attack on Biden's healthcare plan that Kamala Harris originally made at the Democratic debate in Detroit in July

Biden has faced questions about his mental ability to take on President Donald Trump after a series of verbal gaffes.

But questions about his health rose after his eye was seen filling with blood during a CNN town hall debate on the climate on earlier this month.

If elected, Biden would be the oldest person ever to occupy the Oval Office.

He has said he will release his medical records and questions about his health are fair but his campaign has not said when those records will be made public.

After the fight between Castro and Biden, Pete Buttigieg stepped in to call for an end to the bickering.

'This is why presidential debates are becoming unwatchable. This reminds everybody of what they cannot stand about Washington. Scores points against each other. Poking at each other. And telling each other that - my plan, your plan,' he said.

But Castro interrupted to attack him too.

'That's called the Democratic primary election, Pete,' Castro said. 'That's called an election. That's an election. You know? This is what we're here for. It's an election.'

After the debate, the comment attracted trenchant criticism from another contender, Minnesota senator Amy Klobuchar.

She told CNN: 'I just thought "This is not cool." I thought that was so personal and so unnecessary.

'It feels like something Donald Trump might tweet out.'

Bernie Sanders told the network: 'I will be contrasting my view, and my position and my record with Joe Biden's. I'm not going to go after him personally.'