varicella virus actually infect kids with immunity from chickenpox through the vaccine? Kids now receive the varicella vaccine that protects them against chickenpox, but I don't think it provides protection against developing shingles later in life and certainly kids don't generally get shingles (I say generally... I got the shingles 8th grade) But in either case, the virus that causes both shingles and chickenpox doesn't "know" the age of the host either but affects the host very differently.

I don't know if there are other diseases that would be a better example, but certainly the infection rate with covid seems to be less with kids. Saying kids are immune is does not necessarily imply he thinks the virus "knows" the age of the host, although it does show he ignores actual evidence of kids clearly being infected. But in the early days when this was still confined to Wuhan, there was speculation that kids under 5 WERE actually immune since there were no confirmed cases... so the general idea of a virus not infected a host based on age is not totally out of left field.