My girls are now 17 years and 4 months old. They have been wonderful loving pets. Each has arthritis and are taking Metacam. Brandy is starting shots since 2 weeks ago her rear legs gave out and she fell and could not get up. One is starting snap at us when we pick her up in the morning to go outside. Neither moves without limping, both can barely hear, Brandy has bad cataracts and pants due to a weak heart. It appears logical that their end is near. But they look so happy and wag their tails when I am near. How does one know it is time to let the go?