Thank you for contacting Drs. Foster & Smith with your concern.
Ferrets do not lose hair just simply because they are old, there is an underlying issue causing the hair loss. Hair loss can have several different underlying causes, some examples include external parasites (fleas) or hyperadrenocorticism (a disease of the adrenal glands). We would recommend an exam with your veterinarian to determine the underlying cause and most appropriate treatment.
We hope this information is helpful. Thanks again for contacting us.
Drs. Foster & Smith