You need a vet check.
At 15 yrs your baby has a body thats requires more. You could ask for a blood sugar test, increase in thirst and urination could be a sign of diabetes. Or it is simply that as we age the muscles aren't as tight and can't hold the bladder as long.
This might just be old age. Good luck, maybe others have had similar circumstances and can shed light. We have never been blessed with such a long life for our dogs.
Love Them & Lead Them,
~Elizabeth & Doug
Dog Training the Way Nature Intended