It’s important to know the age of your water heater. The average warranty of a water heater is six years. In our experience we have seen water heaters last as long as fifteen years, but letting the heater get this old is a gamble and we do not recommend it. This is an appliance that you should not wait until it fails before replacing it. A water heater going out can range from the inconvenience of not having hot water when you want it, to the tank leaking and causing devastating property damage. Most water heaters in our part of Texas (Katy, Houston) are located in the attic, so you can see how this could get out of control if a leak accrues. If your water heater is ten years or older we highly recommend having it inspected by a licensed professional plumber. To find out how old your water heater is you can find the name of the water heater (usually at the top of the unit) and the serial number (often located on a sticker about half way up on the heater). Look up the manufacturers name online to get their 1-800 phone number. You can call the 1-800 number and give them the serial number and they will tell you how old your water heater is.
Here are a few helpful links for some common water heater manufactures:
A.O. Smith: http://www.hotwater.com/