Might be a difficult question to answer but I guess another way of putting it is; do we know of any examples of early dog burials whereby the age of the dog is noticeably older than their wild counterparts? And if it can be answered- roughly when can we say their lifespans started to become longer thanks to humans?
I am answering this with a knowledge of population biology, not history. An example of an ancient burial really doesn't tell you anything about average lifespan. You need to have evidence from populations. You would have to collect evidence from multiple remains, in both wild and domestic dogs. Possible, but I don't know if it has been done.
Also, it is worth noting that finding a particularly old specimen doesn't tell you anything. The increase in average human lifespan is primarily due to a decrease in the number of infant deaths and deaths at all ages from communicable disease, not advances that extend life of the elderly.