The great majority of people with Alzheimer disease will have developed the condition for unknown reasons but not due to inheriting a familial form of the disease.
Alzheimer disease:
Can be familial or sporadic and, in either instance, can have early (<60years) or late onset
Is the most common cause of dementia in people older than 40 years
Is a pathological diagnosis based on the presence of amyloid plaques and neurofibrillary tangles and cannot be diagnosed with certainty by clinical assessment
The risk of developing the condition increases with age, like other forms of dementia.

Non-familial late-onset Alzheimer disease

There is no discernible increase in risk for the individual if the family history comprises:
No relatives with Alzheimer disease, or
Grandparent only with Alzheimer disease, or
Parent with Alzheimer disease diagnosed before the age of 65 years, and the patient is asymptomatic
and currently several years older than 65
For individuals with a family history of Alzheimer disease, the risk of Alzheimer disease depends on the
degree of relationship and number of relatives affected. In most cases, the individual is more likely not to develop Alzheimer disease.
The risk of late-onset Alzheimer disease among 1° relatives of individuals with probable or definite Alzheimer disease by age 85 years is approximately a 2.5 fold increase over that of the general population.