Well Kashmir turns 6 on November 26 and Zoe will be 10 on December 31. I know that Zoe is a senior, but at what age does a dog become considered a senior?
Here is a link to that very discussion on dogster. I love the chart I brought Bunny ,who is 8.5 years old now, to the groomer on the weekend. When she picked him up, she noticed he had lost some muscle mass, so I guess he is getting up there now Don't tell him that, he thinks he is still a puppy
http://www.dogster.com/dogs-101/calculate-dog-age-in-dog-year s
I would say it depends on the breed of dog. Large dogs have shorter life spans. So what might be middle aged for a smaller dog would be senior for a larger one.
For small dogs it is 9 years, for large dogs it is 6-7 years, according to my vet. Big dogs tend to have shorter lives due to heart complications, but small dogs have plenty of issues too.
A dog's life span will depend on its breed and it is true that most large dogs age faster than the smaller ones. Here is a link that will show you when dogs would eb considered senior depending on their breeds: http://www.petplace.com/dogs/when-is-a-dog-considered-senior/page1.a spx.