The biggest problem to answering this question is determining when the 'Alzheimer's' disease actually started. There is not yet a good test to determine exactly when the disease began. Sometimes it is not even recognized until the brain danage has well progressed and the symptoms are signifigant and undeniable. If you are halfway through a disease process when it is first recognized then asking how long you last after diagnosis becomes rather meaningless.
Saying that people who are diagnoised after age 85 die sooner than people who are diagnoised at a younger age is also of little value because any time you have two people with the same disease and one is in their 50s and one is in their 80s it is almost automatic that the older person would die first.
It IS IMPORTANT to understand that dementia is a terminal diagnosis!
Alzheimer's disease is 100% fatal, there are no surviviors!
So the question about how long somebody might live factors mostly into planning for the future. How long will family member have to provide (endure?) caregiving? The family needs to calculate how long are they going to have to pay for profesional care. Is there enough money available for this? How are any assets going to be managed to be able to provide care and STILL have a life after the AD person is gone?
As for PDXscott and others who describe how to 'live better and longer' with Alzheimer's disease please carefully consider your advice. I have a 62 y/o wife currently in the moderate stage. (Sx came on at 55) I am 54 y/o and really DO NOT want my wife to live another 10 years! Her fate is sealed. My job is to try to survive in such a manner that I can re-establish my life after it is all over.
Nobody has ever survived AD. The only survivors are the caregivers, protect yourself, dont let dementia take two victims.