I didn't think a dog was a senior until it was 10 or older. But what do I know. Oreo is 14 and 1/2 and we think she will live forever because she is the orneriest of the bunch. (you don't MAKE her do anything unless you want bit!) I guess I would also say that I think it depends on breeds. Like with smaller dogs like Oreo they tend to live until 12-14 years or older so their senior age would be different than a large breed who may only live until 8 like a Great Dane. Then 6 would be senior.