There are a number of factors that can determine when a Jack Russell is considered a senior dog.
When the dog reaches seven years of age they are considered senior. Of course, any dog that is 15 years or older is considered a senior dog as well. As a dog ages, he can develop additional health issues which in turn will require more care.
As a dog ages his immune system will weaken and this makes it more likely he will contract illness. Also, senior dogs can develop certain medical conditions that can be life threatening. The simple fact of a dog growing older does pose a major risk for illness.
Older dogs can also become more timid, especially as they grow older. Many senior dogs can develop anxiety or become depressed with the lack of their owner’s attention and the attention of their surroundings. They can also develop arthritis and loss of muscle mass. This can lead to difficulty walking, which will require more exercise and possible medication.