Moderator Dana Bash asked Biden, 81, and Trump, 78, to address voter concerns about their age.

"First of all, I spent half my career being criticized for being the youngest person in politics," Biden said. "I was the second youngest person ever elected to the United States Senate. And now I'm the oldest. This guy's three years younger and a lot less competent."

Biden said voters should look at his record and what his administration's accomplished.

Trump pointed to the "cognitive test" he's taken. In 2018, he took the Montreal Cognitive Assessment, a test designed to detect early signs of memory loss and other mild cognitive impairment.

"He took none," Trump said of Biden. "I'd like to see him take one, just one, a really easy one. Like go through the first five questions, he couldn't do it."

Joe Biden pointed to a plan from some House Republicans to cut $1.5 trillion in Social Security benefits by raising the retirement age as proof that Donald Trump wants to cut the program.

Republican leaders in Congress have backed off slashing Social Security benefits, but many see the current state of the program as untenable in the long term.

When former President Donald Trump was asked what he would do about climate change, he said that the U.S. had "absolutely immaculate, clean water" and air when he was president.

Joe Biden retorted: "He hasn't done a damn thing" for the environment.

Climate change isn't an area where Americans think Trump performed especially well as president. Nearly half of Americans said Trump hurt the country on climate change while he was president, while relatively few Americans - only about 1 in 10 - said Trump's presidency helped the country. About 4 in 10 say he neither helped nor hurt.

They're more likely to see a positive effect from Biden's presidency, but it's not an overwhelming endorsement. Nearly half say Biden neither helped nor hurt the country on climate change, while about 3 in 10 say he helped a lot or a little and roughly 2 in 10 say he hurt the country.

An hour in and here is the Biden campaign spin on what so far has been a rocky performance by the president.

Biden has a cold (but tested negative for COVID-19), the campaign confirms. They haven't offered any explanation for why they didn't disclose this sooner.

Overall, the campaign says they are feeling fine. They agree it was a slow start but say he's found his groove.