Shenk: Alzheimer's is a progressive and ultimately fatal disease. On average, it takes 8-10 years to progress from initial diagnosis to death. It can take half that amount of time, or more than twice that amount of time. I've known of patients to have it for 20-25 years.
Very often, elderly people with Alzheimer's disease will die of a different, unrelated condition before they get to the end stages. Everyone has to die of something, and it's very common for someone to have Alzheimer's for a few years, then die quickly from heart disease or cancer. Alzheimer's doesn't bring on any of these diseases, but it also doesn't protect anyone from them.