Anode Rod:
The most important factor in whether a water heater will last many years is the the condition of the Anode Rod. Not all water heaters have them, the ones that do last much longer than the ones that don't.
Anode rods are made of Magnesium or Aluminum for around a steel wire core. They are screwed in to the top of the water tank. In most cases, water heaters with 5 or six year warranties have one, and tank with 10 or more years have two or more. Commercial tanks can have up to five anodes.
What do they do?
When the water tank is filled, electrolytic process start to eat away at exposed steel. When Anode rods are installed, they are consumed to protect the steel.
What is electrolysis?
When two medals are physically connected in water, one will corrode to protect the other. Anodes are used anywhere one wants to protect metal exposed to water. Metals can be gauged on the galvanic scale of reactivity. When two dissimilar medals are placed in water, the less reactive one will remain intact while the more reactive one will corrode. In cases of water heaters, where the is copper and steel parts the steel will rust faster than if it was along. This is why dielectric unions are used on water line connections.
As in the case with Anodes, they are composed of Magnesium or Aluminum. These medals are very reactive and will corrode before the steel in the tank. Fortunately, they are replaceable thus periodic maintenance will increase the life of your water tank.
How long do they last?
Anode life depends on the quality of the water, tank use, and of course the quality of the tank. In salt water (softened water) anodes corrode faster. Installing water softeners will reduce sediment (good thing) but over softened water will kill the anodes and water tank in less than a year.
When you remove your anode, the condition can tell the story of your tanks life. Rough anodes are normal and its working fine. If you see just a steel rod, you waited to long to change it thus your tank may already be damaged. Changing the anode might get you more life, but it is a gamble.
What do they look like?