There are lots of quality dog foods out now. We feed our dogs a lot of meat. As they age, their kidney and liver function often declines. That means you should feed less protein.
I had a stock dog that developed some problems at age 12. The vet had no advice. I read up on it and changed to some high quality dog food made for senior dogs. She lived another 3 years to 15.
My last Border Collie went backpacking with me at 16. The vet said she had bad liver function but gave no advice. I gave her the same supplements from a health food store that I would give to a human. Her diet was reduced to cottage cheese, canola oil, rice and few other things at the end, but she lived 2 years past what the vet gave her for life expectancy.