If you approximate the youngest age group's data points as a linear trend, it starts well before 2019. After all, they originally were at the same level as the next two higher age groups. So even if you assume that the entire rise after 2020 was due to this cause, it would only explain ~50% of the total effect. And it would not explain at all how older people who were most likely to experience a severe disease (particularly before vaccines) actually show a slight inverse trend, while the age groups in between show barely any statistically significant effect. If you really want to blame covid, I would assume closing schools and mass remote-schooling to protect old people is a much more likely explanation for the trend among the youngest people post 2020. This is the one thing that truly sets them apart from all the other age groups.

Why would that differentially affect young people?

That is a good point. Injection rates were higher with older population. I would say that younger immune systems are more robust, potentially leading to exaggerated inflammatory responses to spike protein, which could heighten neuro-inflammation and cognitive impacts.

Possibly same reason why youngest people had highest rates of myocarditis

You people are suffering from something worse than long covid.