Three Types of Alzheimer's Disease

There are three known basic types of Alzheimer’s disease (AD):

• Early- onset AD is a rare form of the disease that afflicts people younger than 65. Less than 10% of people with AD have this type. Memory loss, behavior changes, and premature aging characterize this type. Because they experience premature aging, people with Down syndrome are particularly at risk for a form of early onset Alzheimer's disease. Early-onset Alzheimer's appears to be linked with a genetic defect on chromosome 14, to which late-onset Alzheimer's is not linked.
• Late- onset AD, the most common form, accounts for about 90% of cases and usually occurs after age 65. Geneticists are working hard to make the connection between genes and AD. There are several suspect genes. Late-onset dementia is also called sporadic Alzheimer's disease. 
• Familial Alzheimer’s disease (FAD). This form is inherited. In affected families, at least two generations
  have had the condition. FAD is rare and  accounts for less than 1% of all cases of AD. People may display symptoms of FAD as early as age 40. The presence of a positive family history in the late onset cases is considered as a risk factor, but a clear autosomal dominant pattern of inheritance is rare.