As you can imagine, the constant grinding of gears against each other as you shift gears results in lots of little bits of metal floating in the fluid. The more there is, the faster the transmission parts will wear out. Transmission fluid also contains detergents and other additives to protect your transmission. These additives are depleted over time, so old fluid doesn't protect as well as new fluid. Your owner's manual will have a recommendation for when you should have a transmission service, or you can bring your car by to determine what services you will need.
Learn more at NAPAautocare.com/preventative-maintenence/