Apart from it being due to her old age, lack of appetite hold various causative factors in a person having morbidities like what she has. It could be a simple underlying viral infection which is very common in elderly in seasonal change, which is causing her not to eat. It could also be her decreased mobility which lets the digestive system and metabolism slow down and therefore decrease appetite. However, significant weight loss can raise eyebrows, and provoke your doctor to consider other major diseases. I advise you to keep track of her weight for a month, and get all her routine health investigation done timely..