Lifespan of dog is most commonly longer than ten years, but one cannot predict it with certainty. There are known cases of dogs who lived for more than twenty five years, but those are rare exceptions.
Smaller dogs reach their maturity faster, within a year or year and a half, but there are also ones who reach it in two and a half years, which is a large dog trait. Also, smaller dogs enter seniority period later and live longer, but there are exceptions such as Bulldog which lives as long as Great Dane, around eight years.
In seniority period, the hair turns grey, especially the hair covering the snout, while the rest of the coat looses its smoothness. Teeth turn yellow and get dull or fall out. Many go blind partially or completely, loose their hearing or vocal ability. A dog gradually becomes torpid, indifferent towards everything that once brought it joy or excited it. But there are exceptions too, some dogs in their late years do not loose desire to sniff around and energetically react.
Dogs actively do not fear death. It does not burden them. If they have serious chronic disease, humans will feel sorry for them, while dogs, as long as they do not feel the symptoms, will act as if they are entirely healthy, because a dog lives in the moment.
There is no precise pattern that compares dog years to human years. It is common reckoning that one dog year equals seven human years, but it is not precise. Dogs begin changing their teeth at three to four months old which is equivalent to five to six human years. First two years of dog's life are not fourteen human years, but around twenty five. After that, every following year equals less than seven human years. While doing the calculation, the size and the speed of maturing and period of growing old should be taken in consideration, which is all difficult to precisely determine.
The chart which should be taken with reserve, but gives decent guidelines in comparing dog and human years:
dog 3 months – human 5 years
dog 6 months – human 10 years
dog 1 year – human 15 years
dog 2 years – human 24 years
dog 3 years – human 28 years
dog 4 years – human 32 years
dog 5 years – human 36 years
dog 6 years – human 40 years
dog 7 years – human 44 years
dog 8 years – human 48 years
dog 9 years – human 52 years
dog 10 years – human 56 years
dog 12 years – human 64 years
dog 14 years – human 72 years
dog 16 years – human 81 years
dog 18 years – human 91 years
dog 20 years – human 101 year
dog 21 years – human 106 years