When to euthanize old dog?

Our 14 year old dog has been pooping in the house lately. He is a 60lb mixed breed who has been the best dog we have ever had. He could go 12 hours alone at home without ever having an accident in the house, now he will be walking along and pooping, I don’t think he even realizes he is pooping. He is very weak in the back legs and can no longer climb stairs and often has difficulty getting up, and has been on medication for pain for about 4 months. He still has life in his eyes and gets very excited to see all of us when we get home. How do you know when it’s time to euthanize?