Alzheimer's at what age?

When I think of the past mom always called my brothers and sisters by the wrong names. Their were 6 of us. And we always answered because we knew who we were. Even if we were in trouble. I wonder if at age 45 she was showing signs of Alzheimer's. I have been surprised by the ages of dementia suffers in communities I have been looking through. Alzheimer's is not just for the aged it seems to effect anyone at any age. Kind of worrisome.