Elderly cats often get kidney failure, and they drink a lot. They also get an overactive thyroid (weight loss).
My little Stella had both, which were detected early enough to be treated - NOT by tablets, which I thought was too traumatic - but by a cream that was rubbed into the skin of her ear once a day, and a liquid which I squirted into her mouth once a day. The treatment cost approximately £80 per month, and she needed blood tests every 3 months.
She was very thin, all of her bones could be felt. She weighed 5lbs. She ate 4 pouches of food a day, plus biscuits. Some days she had fresh, cooked chicken for the 4 meals a day.
She had to be put to sleep last April, due to heart failure and general old age. She was approximately 18, possibly older.
The main thing is that your beloved cat is kept pain-free, warm, cosy and comfortable, in her old age. x