The Woof has a post relevant to my thoughts this week.
In this case the dog lives longer than 20 years. As I understand it small dogs usually live longer than large dogs. Large dogs making it to 20 I think would be rare. Beth's Mozart, a 15 pounder, managed to make it to 20. I think Sasha made it to 10, she was 80-100 lbs. Marshall made it to over 10, weighing in at 37, but then he may have had the "long-dog" back issues like dachshunds.