Your cat is well into her senior days. Yowling is typically a sign of discomfort or other medical issues, especially at night. I would have her checked by a vet, I worked for a vet for a few years and it's pretty common in aging cats. You can read an expert answer here about common causes:
http://www.vetinfo.com/catold.html
"Night-time yowling is a very common problem in older cats. Like many symptoms, it has multiple causes. Many older cats have more than one of the listed problems, so it is important to carefully consider the possibility of all of them in an affected cat.
Hyperthyroidism is the most commonly reported inducer of this behavior and is also the most likely medical cause in a cat that is on the young side when the behavior occurs. This is relatively easy to test for but it can take more than one test, in some instances, to confirm the diagnosis. Usually cats with hyperthyroidism have increased activity levels and are eating more but not gaining weight.
Hypertension (high blood pressure), normally associated with kidney failure, can cause night time yowling. Or at least treatment for hypertension sometimes decreases night time yowling --- it is hard to be sure that the medication doesn't have a beneficial effect separate from the lowering of blood pressure unless accurate blood pressure monitoring is done.
Cats appear to be affected by decreases in their ability to see and to hear in a manner that induces yowling at night. There is a theory that this produces anxiety and that yowling is the result. Some cats do respond to anti-anxiety medications such as diazepam (Valium Rx) or amitriptyline (Elavil Rx).
Cats also seem to get cognitive disorders similar to those experienced by dogs and humans, in which confusion, loss of orientation and other signs of decreased mental ability occur. This may lead to night time yowling, as well. The currently recommended treatment for this is to try selegiline (Anipryl RX). This medication isn't approved for cats (but then neither are diazepam or amitriptyline) and there is less proof that a condition responsive to it exists in cats than there is in dogs.
Sometimes simple things like giving an antihistamine at night, deliberately keeping the cat up and active in the early evening and making sure food and water are easily available at night will help.
I have heard that older people sometimes experience difficulty sleeping at night that doesn't seem to be related to other problems. I think this might also occur in cats. It seems possible that this is sometimes just a complaint about not being able to sleep or a cat looking for a little comfort when it is bored at night and can't sleep. If this is the case I don't know what to suggest except maybe adopting a nocturnal pet to keep the cat company. Of course, that has lots of potential problems as a suggestion, too!
Ruling out hyperthyroidism and hypertension seem like the best first steps, to me. Hope this helps some."
-Mike Richards, DVM 1/26/2000