I know this was addressed to Krikkit, but I hope no one minds if I comment.
I think Krikkit is being modest - I think her long lifespans are very much related to the way her dogs are raised. While genetics do play a part, I think environment has much more to do with the health and longevity of our dogs' (and our own!) lives than we give it credit for.
Is longevity related to good bloodlines? Type of food? That great climate? Your dogs obviously lead very active lives.
IMO, all of the above! Dogs from lines that don't live long are probably more likely to have a short life (those dogs are dying of *something* other than old age, and it's probably at least partly hereditary). Food is a very important factor and the fewer chemicals you pump into your dog's body the better off he'll be. Medications and vaccinations follow this line. As far as climate, well, unfortunately I've never been to Oz, but I'd imagine in general it's less polluted than most of North America (and Europe, for that matter). Active lifestyles are also very important. Most North American dogs (all breeds, not just Boxers) are overweight. Some grossly so. I don't know the statistics offhand, but I know that even being 10 lbs overweight can take years off a person's life - I'd imagine there is a correlation for dogs.
There is a reason that dogs lived longer, healthier lives 50 years ago. Considering the improvements in veterinary care and pre-breeding testing we have today, it can't be only genetics!
Julie