The statement itself is true, up to some change in the covid numbers. Anecdotally there are more young people sick now than when his data were compiled. The crossover point below which flu is more dangerous than SARS-CoV-2 (at least the original strains) is about 14 years old.
Kerpen's conclusion, however, doesn't follow from this fact in any way. Adults are at much higher risk than children from both flu and COVID–19, and of course the elderly are at increased risk beyond prime-of-life adults. Kids don't live by themselves. They encounter parents, teachers, grandparents, and other adults. The idea of allowing COVID–19 to circulate freely among children is sure to promote more breakthrough infections in older persons (that is, Boomers) for whom the illness is dangerous.