Depends on the dog. Scully was probably about 7 or 8 when the arthritis in her elbows started slowing her down. Mulder was still competing in Obedience at 10, and didn’t act like an “old” dog until more like 12.
Chloe just turned 8 and the rehab vet that did an evaluation on her (looking at an old injury) said she is in the shape of a 2 year old.