If you know they’re going to have it twenty years early, then you can try out preventative treatments. You can look at causes. You can get them to put power of attorney in place and prepare for their future.
Why are you so furious about the idea of people knowing?
No one is furious, but it is well established in epidemiology that more knowledge is not necessarily better for the patient. There are psychological harms that occur from the moment you learn that you have a disease that will slowly rob you of your mind. In general, you want fewer harms in your life.