So sorry to hear about your sweet Presley.
I do not think this is 'old age' related, at least not unless that has been diagnosed by exclusion.
With just increased peeing alone (ie, urgency and frequency, but not necessarily increased volume), you'd absolutely want a urinalysis. But with increased drinking and peeing - you should get blood chems (you want to see kidney function), glucose (can be diabetes related), and consider the other symptoms of Cushings. If Cushings is suspected - then the very first test you run just as a screening on whether to proceed further to actual diagnosis is the Cortisol-Creatinine Ratio Urine test.
Symptoms (entire article here)
Some dogs with Cushing’s disease show the classic symptoms, while other show only a few vague symptoms. The classic symptoms are:
--Polyuria/polydipsia (PU/PD)- This is excess urinating and excess drinking of water. It is one of the first signs of the disease, and usually precedes the other symptoms by a significant period of time. Several other important diseases cause these symptoms also, notably
liver disease,
kidney disease
pyometra, and
diabetes mellitus (sugar diabetes).
--Pot bellied abdomen to the point a dog might look pregnant. It is due to hepatomegaly and abdominal muscle weakness (the mechanism of which was described above in the physiology section).
--Thin skin and usually symmetrical hair loss along the trunk. The hair might grow in lighter in color or lose its luster. It might not grow in well at all. Calcium deposits under the skin, called calcinosis cutis, occur on occasion. Secondary skin infections called pyoderma are common also. The skin might also be hyperpigmented.
--Muscle wasting over the head, shoulders, thighs, and pelvis.
Polyphagia- excess appetite. This is often interpreted by clients as being healthy, since most people think of a sick pet as not eating well. In this case your pet is over-eating, which is consistent with Cushing’s.
--Other occasional symptoms include:
Pruritis (itchy skin)- due to secondary bacterial, fungal, or parasitic infections of the skin
Panting- due to affects on the lungs or the respiratory center in the brain
Obesity
Anorexia (poor appetite)
Straining to urinate or blood in urine due to urinary tract infection or bladder stone
Weakness
Depression
Aggression
Lethargy
Corneal plaques
Irregular heat cycles in female dogs
Testicular atrophy in males and clitoral enlargement in females
Emesis (vomiting) due to pancreatitis
Ataxia (incoordination), blindness, circling, and seizures due to a large pituitary tumor or spread of a malignant adrenal tumor
Lameness due to a ruptured cruciate ligament
Intra-abdominal bleeding near the kidneys (retroperitoneal space) resulting in anemia, weakness, and abdominal pain