It is not the most pleasant chapter but it is nevertheless necessary because youth has only a time... One knows, as soon as one opens its door to a dog, as soon as he has an average life expectancy so shorter than ours that irreparably one will see him disappearing too much early. Quite as the human beings, dogs are not equal in front of this phenomenon. It is not so useful to veil us the face. On the contrary, one can, due to our attentiveness, offer a more comfortable old age to our companion and, why not, prolong it of some years.

It is difficult to fix a life expectancy for every dog but it is not rare to meet some dogs with fifteen years and more. If it is necessary to fix an average (all confused breeds), we say that it is situated at about twelve years by taking into account all the parameters and notably the premature mortality (about eight years) due to cancers and cardiac problems. From there,, the importance of a serious and more frequent veterinarian follow-up when the animal sets age.
Certain phenomena will have you to worry and among those, let us quote:

The hygiene of life is important. It will depend of course mainly on the Master. When it ages, the dog has less need of calories; one will thus take care to decrease the meat share replaced by an increase in vegetables, while preserving the same quantity of rice. If the animal is accustomed to the industrial food, one will privilege the food especially conceived for the old dogs. There is also foods adapted to certain case; it will be advised to you by the veterinary surgeon. A last point, but it will have been understood, a dog which knew to preserve a slender pace thanks to the exercise and with a reasonable food will age better!
Despite everything your care and those of your veterinary surgeon, it will always arrive a day when the fatal date will be close... Why prolong its sufferings when nothing any more can be tried? It will be a heavy decision to take but I think that one should not hesitate to shorten its sufferings by having recourse to the euthanasy that will advise you certainly the honest expert. Reassured by your presence, your companion will fall asleep calmly and peacefully...