Alzheimer's disease is a form of dementia
that causes the people that suffer from it to lose memory, ability to care for themselves, and ability to interact with others. The progression of Alzheimer's is quite variable. Some people can live functionally for 10 years after the diagnosis while others are unable to care for themselves after only a few years.
There are many reasons for why someone may progress with Alzheimer's faster than others. The most common reason is a delay in diagnosis because the person with the disease was able to hide it from loved ones despite being very impaired. Thus it is possible that your grandmother was having difficulties that the family didn't notice until they were blatantly obvious. Another possibility is that your grandmother developed anther type of dementia on top of the Alzheimer's, which made her mind decline even faster. A common example is a patient with Alzheimer's that rapidly gets worse because of a few small strokes that went unnoticed.
The most important thing that you and your family can do for your grandmother is to make sure that she has a good primary care physician
that she sees regularly. A good doctor
can make sure that your grandmother and her family have all the resources available to be as healthy as possible at home.