I don't believe I would ever just categorically switch to senior food
because of an age number. I think that cats need a good quality protein in
old age and that many senior foods drop the ball in that regard in an
attempt to cut calories.

Each cats needs should be evaluated separately- some cats put on weight as
they get older, while others have trouble keeping enough weight on. Some
cats have diseases/conditions that mandate a specific diet. It's not as
black and white *IMO* as ___ years = senior diet.

If you describe your cat in regard to his age, health, activity level etc.
I'm sure folks will hep you determine what his specific needs are.