Trump made the claim at a September 21, 2020 rally in Ohio.

You know, in some States, thousands of people, nobody young below the age of 18, like nobody. They have a strong immune system. Who knows? Take your hat off to the young because they have a hell of an immune system, but it affects virtually nobody. It’s an amazing thing. By the way, open your schools. Everybody open your schools.

However, in a taped February conversation, Trump said the virus was five times more deadly than the flu, referring to it as “deadly stuff.”

Trump immediately after went on to claim that “Joe Biden wants to impose a $4 trillion tax site, ban American energy, destroy the suburbs, give free healthcare to illegal aliens” and that the left was going to “indoctrinate your children with poisonous anti-American lies.”

On the day of the rally, the United States totaled 7,067,486 cases and 204,525 deaths.

The New York Times – Trump falsely says the coronavirus pandemic ‘affects virtually nobody,’ as U.S. death toll nears 200,000.
Politico – ‘It affects virtually nobody’: Trump downplays virus threat to young people
Rev – Donald Trump Swanton, Ohio Campaign Rally Transcript September 21
Worldometer – Coronavirus, United States