From what I have read on dog nutrition, a dog doesn’t need special senior food. In fact, often those foods are lower in protein and senior dogs need more protein. I stick with high protein all stage dog food. Plus, I make dog food to supplement the bought. A senior dog may just need less volume. Mine is doing splendidly on Instinct dog food. He is close to 11.
With a couple exceptions, senior dog foods are inferior. Most are too low in protein for seniors; they need a higher protein food. Also, alot of seniors have arthritis going on and grains can be inflammatory so I prefer grainfree.