What truly drives disease susceptibility is an individual's health status and the robustness of his own immune system. This, in return, depends on your diet, protein intake, and micronutrient stores in the body, which allows for proper regulation, proper function, and proper response of the immune system against pathogens. Even people who theoretically never encountered SARS-CoV-2 or other HCoVs (not possible) are immune in a practical sense because a strong and healthy immune system is precisely designed to protect you against new pathogens and new mutations. It is designed to learn, adapt and respond to changes and conditions in its environment. One thing that has been omitted entirely from this conversation is why children and young people have no problems with this virus. The reason for that is simple. It is much easier to fulfill your body's nutritional requirements when you are in a development stage (low bodyweight) and when you are primed for stress, adaptations, and interactions with the environment around you. Kids and young people also have much more active lifestyles, which include movement, exercise, and sun exposure. And yes, this is as simple as that. If it weren't so simple, then no animal on this planet would reach adulthood / old age, as they would be constantly being consumed by "diseases" and pathogens.