I have had many pets over the years and my vet has been my best resource regarding when this is appropriate . She says if the animal is in pain that she cannot relieve , if the animal can no longer keep itself or its immediate environment clean ie . incontinence , or if the animal appears to be distressed , then it is time to euthanize . I had one pet with brain cancer that was not suffering so he died at home , peacefully in his sleep . I had another who did the same of simply old age . I had to euthanize one who went into organ failure she was 20 to spare her the pain of a grizzly death . It really depends on the situation . Have a good talk with your vet about the particular situation of your pet and then heed their advice . Euthanasia is something we must always be prepared to consent to if our pets wellbeing is on the line .