The mattress industry used to tell you to replace your mattress every 8 to 10 years, and later changed that to every 5 to 7 years. You are the best judge of exactly when to replace your mattress in this time frame. Your decision should be based primarily on the condition of the mattress and your comfort, but keep other aspects in mind, too.
Condition of Your Mattress
If your mattress looks old, it probably is old and should be replaced. If there are any dips in your mattress, or if it sags across the middle third or along the sides, it's worn out. When you sit on the side of the bed, does the mattress collapse or does it support you? If it collapses, or if you feel the coils while you're lying down or see any coil or wire sticking out of your mattress, that's a sign it's time to replace it. Any creaking or shifting when you move around is an indication you should start shopping for a new mattress.
Changes in Your Lifestyle
Sometimes a lifestyle change can warrant the purchase of a new mattress. You may be sleeping more or less. You may have started sitting up in bed to read or work on a laptop. If there are two people using the mattress, take into account any changes in your partner's lifestyle that might accelerate the need to replace your mattress. Moving can also put stress on a mattress and shorten its lifespan.
Changes in Your Body
Our bodies change over time, and that affects the longevity of your mattress. If you've gained or lost weight, that can shorten or lengthen the time your mattress will give you proper support. Pregnancy also alters the stress put on your mattress, as do children jumping on the bed. Sometimes our bodies tell us it's time for a new mattress. Your arms shouldn't "fall asleep" or tingle at night. If you've developed pain in your back, it could be the fault of a mattress that needs replacing.
You shouldn't have to look for a comfortable spot in your mattress before you go to sleep. Most importantly, you should wake up feeling you've had a good night's sleep. Even if you have none of the warning signs that say it's time to replace your mattress, you might be surprised at how much more comfortable you are with a new one. You may prefer a harder or softer mattress than the one you last purchased.
Turn your mattress frequently to prolong its life. When you do, and when you change your sheets, take the time to look at your mattress and evaluate its condition. Also take a look at your box spring. When you replace your mattress, you'll want to get a new box spring as well.