We've only fostered and adopted older dogs for years now so all our dogs are seniors or quickly become so. Never used any special food for any of them. They stay on whatever they have been eating that suited them for all the time they have with us. Can't see any point in changing things and risking upsetting them when there is no proven need to do so.

I've had two dogs that lived to nearly 18 & 19 and neither had senior food. Think its just another way for manufacturers to make money out of us caring owners!

I wouldn't recommend senior food, usually it's a bit lower in fat, some are higher or lower in protein (seems they can't make their minds up on which is right) and usually have something added to help with joints which quickly deteriorates and you can't change how much they are getting.
I would stick with what ever suits your pooch and add in something yourself for help with joints, I highly recommend yuMove. Worked wonders for my boy, didn't see any side effects, not expensive and he liked the 'tablets' which I find is half the battle. There are lots of things on the market for joints so might be worth doing a bit of reading and talking to your vet but that's my take on it.

I've had to start looking at the labels for carefully now,
my dog, at 11 is slowing down & needs a lower protein
diet. If they're not using it the kidneys have to work
harder to excrete it (vet advice) hence why she was
weeing a lot. I look for fairly low fat, low protein foods
and treats for her, doesn't matter if they are labelled "senior"
but these ones usually have added
omega 3 etc for joints, you could always give this
separately

In all the years that I have had dogs and that is a lot of years I have never bothered with senior foods or puppy foods. And of course those that feed natural there are no life stages. Unless they get an intolerance to some foods and then it is easy to not give them that food. If your dog is happy on what it is having then don't change. imo.

Interesting. My old collie is around 14 or more now and has always been greedy after five years of near starvation. Now she eats more and I am happy to feed her what she enjoys as often as she wants .. no weight issues. I cook for my dogs and some raw. She needs to feel filled if you know what I mean. Comfort eating at a very basic level. So less protein and more bread etc. For her age she is grand. Oh she has been blind since a year or more also..