Too many vets are keen to put down ailments as a product of old age, mainly because they haven't the will or the ability to investigate, two issues spring to mind , the lethargy and weeing require tests, wto possibilities spring to mind one is kidney problems and the other is hypothyroidism , both require thorough and appropriate tests, NOT bog standard in house ones. The kidney issues can be picked up on urine samples (full out of house) and the thyroid needs a FULL thyroid panel test. I would thoroughly recommend the facebook sites that pertain to dogs with these conditions, they have been life savers for my dogs, the people know more than vets(not difficult) and have been excellent on diagnosing and recommendations. If your dog is diagnosed with either issues, WHATEVER the vets recommend run thro with these sites as they frequently get the dosage and dosing wrong. Thyroid can be helped greatly with simple tablets, kidneys may be much harder..change of diet AND meds. First move find and join both sites Hypothyroidism for dogs, and kidney problems in dogs(maybe worded slightly different) go for Uk sites as they are more appropriate etc. Don't give up on your dog yet...it could be age, but age isn't an illness in itself it just shows up the problems