I think even within a breed, it will vary by dog. I have dogs in the 6-8 age range who starting to slow down. Some who show no signs of aging at all and....

And then there's Hobo. Hobo is 8 now and he is insanely active. His last musher said he was slowing down and that was part of why he gave me Hobo. I am grateful I didn't know Hobo as a puppy. I think he must have been unbearable! There are days even now when he drives the other dogs nuts with his constant movement, barking and energy.