In my opinion senior cat is the one older than 10, but of course it is only my opinion.
Cats in this age are much calmer. Make sure you take your cat to the vet and do a proper check up just to know what things you should avoid, what food to give etc etc. here are some good food brands for the cats without teeth https://www.catfoodpoint.com/...-cats-with-no-teeth/ (not sure if it is your case but possible)