If your older cat is acting weird, it could be a mental disorder... even Alzheimer's disease... according to a news item posted at Discovery News. (See Alzheimer's in Cats)
Dementia isn't just for humans anymore... more than half of domesticated cats over age 15 have been identified as having some form of mental disorder, possibly due to their close association with people, as they often are exposed to the same kinds of environmental and nutritional deficiencies as their owners.
The good news is that many cat health problems may be mostly preventable with proper diet along with physical and mental stimulation throughout their lives. In other words, feed nutritionally complete foods, keep kitty away from hazards, and play with her to keep her mind active.