Alzheimer’s  is a complex disease that progresses over many years, like diabetes , heart disease , and other chronic conditions. A number of factors may increase or decrease a person’s chances of developing the disease. These risk factors include age, genetics, environment, and lifestyle. The importance of these factors may be different for different people. Some risk factors can be changed or controlled while others cannot.
Research shows that Alzheimer’s disease causes changes in the brain  years and even decades before the first symptoms appear, so even people who seem free of the disease today may be at risk. Scientists are developing sophisticated tests to help identify who is most likely to develop symptoms of Alzheimer’s. Ultimately, they hope to prevent or delay dementia in these high-risk individuals. (See Finding Out Who Is At Risk )
Age is the best known risk factor for Alzheimer’s disease. The risk of developing the disease doubles every 5 years after age 65. Alzheimer’s becomes increasingly common as people reach their 80s, 90s, and beyond. These facts are significant because the number of older adults is growing. The U.S. Census Bureau estimates that Americans 65 and older will grow from 13 percent of the population today to nearly 20 percent in 2030. The group with the highest risk of Alzheimer’s—those older than 85—is the fastest growing age group.
The more researchers learn about Alzheimer’s disease, the more they realize that genes play an important role  in its development. Scientists have found genetic links to both early-onset and late-onset Alzheimer’s disease.
Early-onset Alzheimer’s is rare, accounting for only about 5 percent of people with the disease. Its symptoms usually appear when people are in their 30s, 40s, and 50s. Most cases of early-onset Alzheimer’s are familial, caused by mutations (permanent changes) in one of three known genes inherited from a parent.
Late-onset Alzheimer’s disease, the most common type, typically becomes evident after age 65. The causes of late-onset Alzheimer’s are not yet completely understood, but researchers have identified several risk-factor genes. One of those genes, called apolipoprotein E (APOE), has three forms, or alleles (ε2, ε3, and ε4). One form, APOE ε4, increases a person’s risk of getting the disease. It is present in about 10 to15 percent of the population. However, carrying APOE ε4 does not necessarily mean that a person will develop Alzheimer’s, and people without APOE ε4 can develop the disease.
Scientists have identified a number of other genes  that may increase a person’s risk for late-onset Alzheimer’s. More studies are needed to assess the role these genes may play and to search for additional risk-factor genes. Knowing about these genes can help researchers more effectively test possible treatments and prevention strategies  in people who are at risk of developing Alzheimer’s—ideally, before symptoms appear.
Genetic testing cannot predict who will or will not develop late-onset Alzheimer’s. Currently, it is used only in research settings and for people with a family history of early-onset Alzheimer’s disease. This type of testing is not recommended for people at risk of late-onset Alzheimer’s. It is not conclusive, and its primary value at this point remains in research settings.
To Learn More
The NIA’s Alzheimer’s Disease Education and Referral (ADEAR) Center answers questions and provides free publications about Alzheimer’s disease and other types of dementia. To learn more about genetics, see the Alzheimer’s Disease Genetics Fact Sheet .