There are the risk factors Zapatista, hope they answer your question...
Age: The risk of developing Alzheimer’s increases with age. One out of every 10 persons 65 years and older is a victim of Alzheimer's disease, although early-onset victims may be in their 40s and 50s. Approximately 20 percent of Americans between the ages of 75 and 84, and almost half of those 85 years and older suffer from Alzheimer's disease.
Genetics: Early-onset Alzheimer's has been clearly shown to be genetic in origin. While mutations on chromosomes 9 and 19 have been linked with late-onset Alzheimer's (the most common form), not everyone with the mutations develops the disease. The relationship between genetics and late-onset Alzheimer's is not fully known.
Other: Some studies have implicated prior traumatic head injury, lower education level, and female gender as possible risk factors. However, at the present time, no definitive causes have been identified.