Aww bless, she's a grand old age, isn't she?
I love old cats!
The others here are right - get her a thorough health check, and check for deafness or hearing loss (sometimes in one ear before the other).
The indicators of "cognitive decline" (they don't call it senility in animals) are as follows (and apply to both dogs and cats - also bear in mind that each point refers to an increase in the specified symptom beyond what is normal for that individual animal):
Unexpected mood swings / unpredictable behaviour
Disorientation (going to the wrong door to be let out; standing at wrong side of door; appearing to be lost or confused in familiar surroundings; difficulty in locating trays)
noticable change in sleep pattern - in cats usually waking at odd times relative to normal routine; wanting in or out at strange times, "asking" to be fed at the the wrong times of day etc
confusion or delayed recognition of familiar people or other animals
increased co-ordination problems
increased vocalisation in absence of impaired hearing
marked change in appetite in absence of any health problems
unexplained aggression
One or two of these is not a problem, but if an animal has more than half of them, in the absence of any health problem (VVV important to check for possible medical problems first), then they are experiencing cognitive decline.
Switching to a good quality senior food, giving omega 3 & 6 supplements, and devising some new forms of mental stimulation (hiding food for them to find, introducing new games, new toys, etc) can improve things.
However, cognitive decline can be slowed but not stopped entirely - we all get a little eccentric as we get old, and so long as her quality of life and health is still good, a few odd little habits are not too much of a problem.