Firstly, I would like to point out that relative old age in dogs and cats is a state that can be made quite comfortable for the animal, in a similar way to humans.
A comprehensive clinical examination is indicated, in addition to a complete blood test. Other tests may be necessary depending on physical health.
Weakness concerning essential organs, liver, kidney, pancreas - and joints - may be corrected by an appropriate diet and/or medication.
Excess weight is sometimes a cause for lethargy - owners are often surprised at the increase in liveliness subsequent to a correctly adapted diet - some animals even become almost frisky again.
I think the owner should ask him/herself the following question: is my animal in good health generally?
If not, what do I think is wrong? We have mentioned lethargy; is he/she frequently out of breath, having joint problems, digestive problems, skin problems etc. Treating this can often improve comfort.
As to the final decision for euthanasia, all I can say is, you, the owner will usually feel and know when the time has come. I like to talk about "Quality of Life". Is the animals' life really worth living? This is sometimes very difficult to answer. If the animal is still eating and digesting well, the decision for euthanasia may be impossible to make, even if sight and hearing are strongly dimished and the animal is in a near vegetative state.
When it comes down to it, only you can make the decision.
From Riviera Reporter 117 - Oct/Nov 2006