Your mattress probably sags because the materials are worn out. Over time, comfort materials—including fluffy pillow tops or foam layers—develop lasting impressions where you lie. Sagging is inevitable, although high-quality materials take longer to form indentations than lower-quality materials.
While age is usually the cause of a sagging mattress, there are other potential reasons your bed might sag. For instance, softer mattresses sink in more than firmer beds, even when new. And the more body weight, the deeper the sagging. If you have a bigger body type, consider a firmer mattress for greater support and lift.
Finally, your mattress can sag when on a poor foundation. You want to ensure that you place your bed on a bed frame or foundation that fits the manufacturer’s recommendations. Old box springs and frames with slats spaced too far apart can cause your mattress to dip.