There are various possible reasons why a gerbil loses hair, ranging from advancing years to parasitic infection. If your gerbil's fur is thinning or balding, it can be a symptom of a health problem, and so should be investigated and treated.
Is your gerbils' coat nice and shiny, or is it looking a bit thin in places?
The place where the hair loss is occurring gives a clue to its cause. For example, hair loss on the belly in association with a lump, it suggests an abscess or tumor, while hair loss on the face or around the mouth could be due to bar rub.
One of the common causes of patchy fur is parasites - if your pet is suffering from parasites such as mites, they will itch and scratch, which could be wearing away their fur.
If you are sure that the hair loss is not due to old age, and there’s no parasitic infection, take your pet to the vet for an accurate diagnosis. Your furry friend could be suffering from some form of disease or nutritional deficiency.