We had to put our 16 year old second hand shetland sheepdog to sleep over the summer...he survived a horrible case of separation anxiety when we got him at the ripe old age of 9 (I really was totally against getting this dog....but my wife is an easy mark...SOOOOOOO glad that we did end up taking him), he survived a cancerous tumor on his leg (same one he used to bite when his old owners would leave), then survived us introducing a shih tzu puppy (to keep him young), and finally got through a couple mini strokes a year and a half ago. I thought that this dog would live forever...over the summer he started having the mini strokes again and more frequently and lost his ability to get around, started having accidents in the house, lost his appetite, and lost his "spunk". We knew that it was time, which broke both of our hearts....but it was almost like he knew how hard the decision was for us and gave my wife a kiss while he was going...that was his way of letting us know that we did the right thing and that he thanked us for not letting him suffer.
Aside from watching my mother die of cancer, that was the hardest thing that I have ever had to do in my life. Animals mean so much to my wife and I that you sometimes forget that you will outlast most of them and may have to make decisions like that.....that said, my house will NEVER be without at least one dog (currently have a sheltie pup and our shih tzu....which is the greatest dog that ANYONE has ever owned...if you would like stories, I can provide a LOOOOOOOONG list of what makes him so great!).
If he is still enjoying the little pleasures such as the pork chop and is still able to go about his routines....I'm not sure that it is time (you will know when it is time...you know your dog).