
Two of my bitches had their last litters at 6 1/2 years old at which point they were still in their prime.
It was not unusual in the USA and in Norway for bitches of my breed to still be having litters at 9 or 10 years of age, I assume as they were busy in the showing and hunting field until then. I think the KC rule of below 8 is about right though. The advantage of breeding from older animals is that their health status is pretty apparent and if they are fit and healthy at that age then they are unlikely to develop any of the hereditary ills.
I know a breeder whose policy was to breed from a 3 year old bitch for her first litter to an older stud, and then let her have another litter at 7 possibly to a younger stud. This way she hoped to improve her chances of ensuring she was breeding from proven healthy stock, especially when linebreeding.