My dog is getting older but when would he be considered senior?
I know that 1 human year equates to 7 seven dog years.
8 years, would be a good average to concider your dog as "old" or "senior" but there are no real fixed age. Just the difference between the races' lifespan makes it easy to understand. Small dogs tend to live between 15 to 20 years as big dogs will live between 12 to 15.
The reasons why I say 8 years old is the threshold is because that at about this age that dark dogs will start having gray hair. At that age, the dog's diet should be changed a bit and the adult food should be changed to senior food as per your dog's favorite brand will most likely point out.
When is a human considered to be old? The answer really depends on the individual.
The seven year rule is old and inaccurate. There are several other methods that take the life expectancy of dogs into account better, eg one dog-year equals five humans-years for small dogs and seven for bigger dogs. And then there are even some nonlinear metrics in determining a comparable age value.
If you and your dog think that he is old, ie he behaves like an old dog, treat him as such. I don't think it is good to treat a juvenile and active dog of high age as if he was a broken senile. Of course you still have to be careful as older dogs, no matter how active they are, will still have age-related problems and are more prone to injury than younger dogs.