If you have noticed that you don't wake up full of energy or don't feel well rested in the morning, that maybe a sign that you need a new mattress. The general lifespan of most mattresses is 10 years, so if you cannot recall when you last purchased a mattress or if it has been over 10 years, chances are you need to start looking for a new mattress. One great resource is http://www.mattressoracle.com where you can find a wealth of information on all of the various types of mattresses as well as reviews of some of the top mattresses in each category.
Another thing to keep in mind, is even if your mattress is not worn out and you still could get many more years of use out of it, if you have either too firm or too soft of a mattress, and you aren't sleeping well, then it still may make sense to purchase a new mattress. Of course there are other options, like purchasing a mattress topper to get a few more years of use out of your mattress, however the best course of action is generally to replace your mattress. Two of the most common issues consumers have with their existing mattress, is that it's either too soft, leading to back problems. Or their mattress is way too firm, leaving them tossing and turning throughout the night, trying to find a comfortable sleeping position.