Difficult one. As a creaky old geriatric myself I'd like to keep going as long as I can and it's the same for the dogs. You've got them for life in my book. If you bring on a pup to take over the bulk of the work let the old one keep going as long as they have the will to try. The sort of stuff I (used to) do will one day be too much for the heart and lungs but better than being left behind in kennel.