I have worked as a neurologist in old age homes and chronic hospitals for 30 years, and over this period I have seen thousands of older patients with dementias and other degenerative neurological syndromes. A high proportion of these patients have steadily and sometimes rapidly advancing dementias of the type we now tend to call Alzheimer disease (AD) (although this term was used in a much narrower sense when I started seeing such patients in the mid-sixties).
Sacks O. A Neurologist's Perspective on the Aging Brain. Arch Neurol. 1997;54(10):1211-1214. doi:10.1001/archneur.1997.00550220023008