MayBee70
Thinking about 20-29 year olds. Maybe it’s because their immune system is still working pretty well at that age (without googling am I thinking of the hypothalamus gland?). It starts to drop of after that, doesn’t it, unless you keep yourself hyper fit. Many will have continued to mix with friends and have been exposed to small amounts of the virus over a period of time but won’t be exposed on a daily basis in the way that schoolchildren are. This is a baffling, complicated virus. I remember doctors at the beginning saying they’d never come across anything like it and that hasn’t changed. We’re all playing Russian roulette with our lives while this virus is still with us.
The immune system doesn't stop people being infected by a novel virus, however robust it is (and whatever the quacks say). However, it could be that their systems are generally in good shape and that they hardly noticed that they were infected.
Increasing age is associated with an increase in PRR genes and ACE2 receptors and maybe some other proteins, which make it more likely that the human body will be affected. The reasons aren't properly understood. Moreover, younger people are less likely to have so-called lifestyle conditions such as diabetes, lung disease and cardiovascular conditions, which mean the body is vulnerable to infection.
What's happening in schools is a result of almost no mitigations. Pupils have been forced to sit near each other in poorly ventilated classrooms and it was forbidden to wear masks. They have to go to school even if a family member is infected and close contacts aren't required to self-isolate. Even young adults don't have to work in those conditions.