Not asking for the gory details but what can or cannot your dog do? You need to make a list of two columns and if the negatives win out then it should be the time to let your dog 'rest'.
Does the dog have trouble walking (even short distances to go outside for relieving itself)? Severe arthritis or hip problems?
Does the dog have a modest appetite? Is the dog severly under or overweight?
If the dog was a big player, has the level of play changed dramatically?
What of sight and hearing? How are they affected? Does the dog have temperamental changes because he/she can't identify friends from strangers?
How I'd the dog's coat? Is he/she going bald or have severe rashes or itching problems? Does he/she get too cold during the cold nights? Etc...?
If the dog is suffering from pain and other symptoms then you should let the dog go. Take him to a vet and stay by it's side at the end. This act of mercy is and act of compassion and kindness.