When it seems they don't have any quality to their life anymore because of the pain and suffering of old age and the issues that come with it.

Whilst they are still able to enjoy aspects of their lives and show it, they deserve to stay alive to enjoy them. Painkillers and treatments can help where arthritis and such are concerned. Difficulty getting up is common and hearing loss also. Some dogs live perfectly happy lives never being able to hear so thats not a big problem where quality of life is concerned.

If the dog just looks depressed and doesn't enjoy anything anymore due to not being able to do it or feeling too ill to do it, then it is time to let them go with dignity.

Sometimes, it is harder to describe in words when you know. When the time comes, you tend to just know.