It's not uncommon for cats to become more aggressive as they age, either toward people or other pets. It is, however, important that you have your cat checked by your veterinarian. There are many medical problems that can lead to an increase in aggression, including arthritis pain, a loss of hearing or vision, and a number of diseases effecting the nervous system. If there are any medical conditions present, treating them can help to change your older cat's behavior. Certain stresses or changes, such as a move, can also lead to aggression.