Alzheimer's disease occurs in two forms - early-onset and late-onset. The early-onset form of the disease is very rare and mainly affects people in their 30s, 40s, and 50s. This form of the disease has been linked to three different genes and has been observed in only 120 families worldwide; individuals who carry one of the early-onset genes will most likely develop Alzheimer's disease.
Much more common is the late-onset form of the disease, which occurs after age 65, and accounts for more than 90 percent of all cases of Alzheimer's. It is unclear whether a genetic abnormality, environmental factors, or a combination of both causes this form of the disease.
What is known for certain is that a person's risk of developing Alzheimer's at any given age appears to be increased if he or she has an immediate relative with the disease, such as a brother, sister, or parent.