According to the DSM-5 the prevalence of neurocognitive disorders (dementias) is strongly associated with aging. In industrialized nations the diagnosis of dementia ranges from between 5% - 10% in individuals in their 70s. This risk increases significantly as people age with most sources reporting a sharp increase for every decade after the age of 65.
According to census data in the United States, of everyone in the US:
- About 7% with the diagnosis of Alzheimer's disease are between the ages of 65 - 74 years old.
- About 53% are between the ages of 75 - 84 years old.
- About 40% are over the age of 85.
- Less than 1% are younger than 65.
Although there is a slight decrease in the percentage of people over the age of 85 with Alzheimer's disease compared to those who are between the ages of 75 - 84, this may be a bit confusing. This happens because many people with Alzheimer's disease die well before they reach the age of 85. If more people lived longer with Alzheimer's disease who were in their early 80s, then the percentage of individuals over the age of 85 with Alzheimer's disease would be much higher. The vast majority of people who develop Alzheimer's disease between the ages of 65- 80 do not live to be 85 years old.
The DSM-5 suggests that the percentage of dementia that is attributable to Alzheimer's disease ranges from between 60% to 90% depending on the setting that the disease is diagnosed in and the criteria used to diagnose dementia. The organization, Alzheimer's Disease International, suggests that overall Alzheimer's disease accounts for 70%-75% of all dementia cases. However, when the brains of people who are diagnosed with dementia are analyzed after they have died, the findings indicate that they often have many different potential causes to their dementia. There are probably many factors that contribute to the development of dementia in most people.
When one reads about the "causes" of Alzheimer's disease one should understand that the use of the word cause is not appropriate given the current understanding of how dementia happens and develops. There are no guaranteed identified causes of Alzheimer's disease in any single person. Instead, competent researchers and clinicians refer to certain types of risk factors that can increase the chances that one may develop Alzheimer's disease.
A risk factor is some type of condition or experience that can increase the chance that a person might develop a certain disease or disorder. However, it does not guarantee that they will develop the disease or disorder. When people read about the causes of Alzheimer's disease, what they are reading about are certain risk factors that can increase the chances of developing the disorder. Risk factors are additive, meaning that if you have more than one risk factor the chances of developing the disorder or disease increases compared to having only one risk factor. The more risk factors a person has the greater the chances are that they will develop the disease or disorder, but there is no guarantee that this will happen.
As one can see by the above description of the data from the U.S. Census, the major risk factor for the development of Alzheimer's disease is increasing age. The chances that a person will be diagnosed with Alzheimer's disease increase dramatically as a person reaches the age of 65 and over. However, simply getting older does not cause a person to develop Alzheimer's disease. In fact, the majority of people that do get older do not develop Alzheimer's disease. For instance, the DSM-5 reports that the prevalence of dementia overall is about 5%-10% of people who are in their 70s and only about 25% of all people who are older than 70. Thus, aging alone is not a cause of Alzheimer's disease. However, getting older increases the risk that someone might develop the disorder.
Researchers report that the development of any form of dementia is due to the interaction of many factors. Thus, as a person gets older there must be other factors that interact with the aging process that result in an increase in the chance to develop Alzheimer's disease or other forms of dementia. In the second part of this article several of these additional risk factors will be discussed.