Fortuno , - we know in general that very young people have strong immune response to pathogen ...this is why they not having as many serious diseases ,
still sometime very young people developing sometimes also very serious problems ...

With age our immune response is changing , as many other things ...(((

With transplants , patients are placed on some immunosuppressant , not to reject particular organ ,

But patient 20 years old , or 50 years old or 80 , the dose ls different..

My aunt who had kidney transplant at 80 , required very low dose...

The younger is patient the higher the dose....this is usual rule...

So this is basic explanation why diseases are not as severe to young ...but still not always !!!!
As we know nothing is 100% in medicine ......