Alzheimer's disease is considered to be early-onset when it occurs in people younger than 65. Approximately 5 percent of people with Alzheimer's disease, which is the most common form of dementia, first show symptoms in their 40s or 50s, according to Alzheimer's Association.
Early-onset Alzheimer's is often undiagnosed because physicians don't tend to look for the symptoms in younger patients. As of 2015, doctors do not know what causes early-onset Alzheimer's disease. Younger patients experiencing memory problems should keep a log of their memory and cognitive difficulties to show to a doctor. Further diagnosis typically involves memory tests, neurological tests and brain imaging, as stated by Alzheimer's Association.