The National Association of Home Builders has put together an excellent study on the life span of building materials, including appliances, wiring, roofing, etc.
You should refer to your manufacturers recomendations as to service requirements. Everything has a life span.
Knowing the life span of appliances can help you plan their timely replacement. There are many rebates and tax credits for replacing with more energy efficent items.
As to water heaters, annually draining and fushing is required by the manufacture, many building code require that the pressure/temperture valve is checked to insure it's working properly. The annode rod should also be checked and replaced as needed. Proper combustion of gas fired water heaters should be inspected for along with proper venting. Water heaters require maintenance just like everything else and should be done on a regular schedule.