When answering the question "How Do I Know When It's Time For A New Mattress", there are 3 primary indicators that it is important to note when trying to decide if it is time to invest in a new mattress set. Mattress age. Most mattresses built in the last twenty years have been built with planned obsolescence in mind. Comfort is usually all but gone within 7-8 years. Sagging. If your mattress has a body indentation greater than 1” it is likely that you are sleeping on a worn-out mattress. You are always uncomfortable or in pain. Whether the mattress is worn out or not, being uncomfortable or in pain is not conducive to a good night of therapeutic sleep. It may be time to try something different and new.