Congratulations! Goes to show age is just a number. The working relationship between dog and handler is very complex, and difficult to understand if you haven't walked that path in person...but they do tell you when they have had enough.
The corgi herding community had a sad week....one of the greats of the breed, a DC (dual champion, both conformation and herding) crossed the Bridge at the age of 14 1/2. I count myself lucky to have seen this dog (and her litter brother, also a DC) run many times, including several runs at 11+ years of age.