Over age 85, nearly half begin to show signs of dementia. Less than ten percent of those with the disease inherit it through the family. An even much smaller number have a strongly inheritable form of the disease which seems to strike at a very early age. Most of those who get Alzheimer's disease get it without family predisposition, and get it in their 70's, or more likely in their 80's or 90's. Those with strong family history, those who have someone with Down's syndrome in the family, and those who carry a certain type of the cholesterol-carrying protein ApoE in their blood are more likely to get the disease.
Some factors decrease the likelihood.
Those with higher education are less likely to get it. Women who have taken estrogen may be less likely to get it. To avoid the disease,stay young, pick your parents well, get well educated, and if you have to take some arthritis medicines or are advised to take female hormones for some other reasons, count some possible protection against Alzheimer's disease as one potential benefit of your treatment.