I'm not sure...but I've heard anywhere between 8 to 10 or 10 to 12 years is when they are considered seniors. But little ones can also live much longer than the large breeds...16, 17, 18 years.
Sumo is 11 years old and he still like to run zoomies in the yard.
I would actually consider a dog senior if he is already around 10 years old or more. I guess it would also depend on whether the dog is from a small or large breed, though.