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
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.