Average expectancy for Jack Russells is about 15 years. If your dog is in good health and not overweight, there's a good chance of living beyond that...extra weight can take a couple of years or more off of any animal. Remember that the average is just that...an average; average human expectancy is just over 75 years, but most people expect to live past that. My last dog died at 15, but he was a Redbone coonhound, a breed with an average expectancy of 12 years. The oldest small dog I know was 19 when she died, so it really all depends on the level of care the dog has had and its current level of fitness, as well as the absence of serious health problems.