So many possibilities for error... Is every case of dementia diagnosed accurately? Is there a difference between professions how likely their dementia is to be

So many possibilities for error... Is every case of dementia diagnosed accurately? Is there a difference between professions how likely their dementia is to be diagnosed accurately? Is there a correlation between dying early and being pre-disposed to have developed Alzheimer if having lived longer? If there is, then the "correction" for age is not correct, because the correction assumes the fraction of the group that died early were equally likely as the rest of the group to develop the disease if they had not died early. I don't know anything about Alzheimer but it doesn't seem unthinkable that the factors that cause people to die early also could also play a role in causing Alzheimer.