The candidates were asked what they could do to help Americans battling the opioid crisis in the U.S.

After being prompted to respond to the question, Trump said that during his term as president they had "great equipment" and drug-detecting dogs to find illicit drugs at the border.

Biden said the U.S. needs machinery that can detect fentanyl and more agents at the border, and added that the U.S. is coming down "very hard" on countries in Asia that are producing fentanyl.

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."

Biden had a poor start to this debate, and if he ends up being viewed as the "loser" tonight, he'd be just the latest incumbent president to go through this. Incumbent presidents like Ronald Reagan in 1984, George W. Bush in 2004, Barack Obama in 2012 and Trump in 2020 all lost 2 to 3 points in the polls after their first debate.

Reagan, the last president to face real questions about his age -- he was 73 in 1984 -- had a meandering performance that prompted questions about whether he had gotten too old for the job. Reagan shot back in the second debate against Democratic nominee Walter Mondale, who was in his mid-50s, with the famous line: "I will not make age an issue of this campaign. I am not going to exploit, for political purposes, my opponent's youth and inexperience." Reagan, however, also had a roaring economy, which helped him win a landslide reelection victory.

-Geoffrey Skelley, 538

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.

-ABC News' Mary Bruce and Molly Nagle

Millions of eyeballs will be on Thursday's debate -- here's what to watch for as Biden and Trump face off.

Will any gaffes or knockout punches break through? Conversations on policy are unlikely to break through, but viewers could remember a serious lapse or a zinger that lands.

Will character or policy rule the day? Both sides have lobbed personal attacks -- Biden has dubbed Trump a "convicted felon" and threat to democracy, and Trump has essentially called the president, without evidence, the leader of a crime syndicate. Will the debate stay focused on issues like abortion, immigration and the economy? Or will the two candidates opt for a more scorched-Earth stance?

Typically, a sitting president is debating a governor or senator, with the incumbent's record the main focus. But Trump also spent four years in the White House. Whose record will get more scrutiny?

The debate is happening uniquely early in an election cycle. It remains unclear if any big moments will be washed away by November, or if it'll help set the tone heading into the summer.

The debate is also featuring a novel format -- microphones will be cut off when candidates aren't recognized to speak, and there will be no audience to feed off of. That could deprive Trump of the energy he seeks, helping Biden. But the microphone cutoffs could help "enforce some discipline" on Trump, said GOP strategist David Kochel.

Read more here.