All good things must come to an end. At some point, your mattress will begin showing signs of wear. And at some point after that, you should start thinking about replacing it with a new one that matches your needs.
The question of how often a mattress needs replacement is one on which the mattress industry has been increasingly vocal in recent years. For a long time, the industry was content with the notion that a mattress should be replaced every 10 years or so. Recently though, large mattress retailers like Mattress Firm started investing millions to advertise the idea that you should "replace every eight" years. Now, mattress retailers are banding together to promote the idea of replacing your mattress every 5-7 years. But wait, didn't you just purchase a mattress with a 20 year warranty...?
The truth is there is no magic number for when to replace your mattress. Much more important than the exact age of your mattress are the specifics of how well your mattress is holding up, and how well you are sleeping on it. For example, if you notice that you are tossing and turning a lot during the night, or that you have new aches and pains when you wake up, these are good indicators that you should be thinking about your next mattress purchase. Likewise, if your mattress is not holding its shape any more, this is a likely sign that it needs replacement. On the other hand, if neither of these are a problem, you don't need to be in a hurry to replace your mattress (especially not on account of dust mite fears).
That said, most experts do recommend that a mattress be replaced at least every 10 years, even for higher-quality mattresses. After ten years, it is normal for mattresses to show signs of wear, such as loss of support and comfort, which may interfere with your quality of sleep or how your body feels upon awakening.
One thing that is very important to understand is that the length of the warranty on the mattress has absolutely no relationship to the expected lifetime of the mattress. Mattress warranties are simply a marketing gimmick designed to sell more mattresses, without providing any meaningful accountability that can be tied to longevity. As such, whether your mattress is still under warranty (or long past its warranty) should not be a consideration as to whether it's time to replace it.
If your mattress is showing signs of wear but you’re not ready or able to replace it just yet, there are some cases when adding a mattress topper can extend the life of your mattress a bit longer, however there are many other cases when this is not a good investment.
In particular, you should consider a topper if your mattress problems are rooted primarily in the loss of pressure relief. Once the upper comfort layers of a mattress have compressed over time, they will no longer provide as much cushioning or pressure relief, which you might experience as pain or tingling in your hips or shoulders. If the underlying support core is still holding up well, then adding a topper can be an effective (albeit temporary) solution to this problem. Just be mindful that the rest of the mattress will eventually give way too, and factor that into your calculation of whether to invest in extending the life of this mattress or put that money toward a new mattress.
On the other hand, if your mattress is suffering from poor support, you should avoid the temptation to add a pad or topper -- it will likely be a very poor investment, even as a temporary solution. Poor support (aka, sagging) is typically the root cause of back pain on a mattress, and unfortunately it can not be remedied by a topper. Back pain is best alleviated by a mattress that supports your spine in a neutral position, allowing the muscles in your back to relax while you sleep. Because the top few inches of padding generally do not affect the support profile of the bed, your spine will soon find itself back in the same (painful) position, even after you add the topper.
Likewise, loss of comfort due to body impressions is another problem that can not be remedied by a new mattress topper. Though it may seem to have a smoothing effect at first, a topper will soon conform to the shape of the mattress underneath it, leaving you with the same (or perhaps even worse) body impressions that you had before.