According to alz.org to test if one has Alzheimer's disease, a thorough medical history, mental status testing, a physical and neurological exam, tests (such as brain imaging) to rule out other causes of dementia-like symptoms would all be looked at by a profesional. Genetic counseling is an option as Alzheimer's is a genetic disease, however it dose not always show clear results.
Approximately 5.5 million Americans are living with Alzheimer's. Approximately 5.3 million or 96% are 65 or older. This shows that Alzheimer's is definitely more common in older people as one in every ten or 10% of the population suffer from it. According to genetics home reference it is also more common with people who have Down syndrome.