Whether or not your dog is considered elderly depends of the size, age, and breed of dog, and also (like a person) on the attitude of the dog.
Typically, smaller dogs live longer than larger dogs and mixed breeds, longer than purebreds.
Although these are vastly oversimplified, this chart shows when your dog may be considered a “senior” dog.
Dog size (pounds) Age considered Senior (Human years)
Under 20 9-13
Over 90 6-9