Cats vocalize (meow) for many reasons. Some cats meow when they want attention. Others meow when they are hungry. In some cases, cats vocalize when they are agitated, stressed or afraid. Many cats vocalize for no reason that humans can ascertain.
There is one medical condition that quite famously causes increased vocalization in eight-year-old cats: hyperthyroidism.
Hyperthyroidism is a disease of cats that in my experience develops most often between seven and ten years of age. It is a disease of the thyroid gland. The thyroid gland is located in the neck and it regulates body metabolism. The disease causes the gland to become hyperactive. Dogs almost never develop hyperthyroidism.
Hyperthyroid cats may lose weight, drink large amounts of water, vomit, produce diarrhea, have poor quality hair and vocalize excessively. Internally, the disease causes high blood pressure, heart disease and kidney problems. It may also lead to blindness.
Hyperthyroidism can be diagnosed with blood tests. It can be treated with medicines or an injection of a special type of iodine.
I recommend that you have your veterinarian assess Martha. The vet will want to run tests. If the tests do not show a cause for her increased meowing, then the change may be due to personality and temperament changes as Martha matures.