What is the life expectancy of my border collie?

Michael is his name and protection is his game. Michael was 12 years old on May 16th, 2010. compared to a human, thats 84 years old. Michael is healthy, fed the best dog food and routinely taken to the vet for a checkup. his muzzle is turning grey and i know he has arthritis in his left knee. so, how much longer can i expect to have my dear friend Michael around? many more years, i hope. he is truly loved.