What ' s the difference between a porcupine and a BMW ? BMWs have the pricks on the inside ! <| sep |> Not a doctor or a matress expert but am someone more or less interested in material science and the first thing that comes to mind is how well the metal springs are at returning to their uncompressed state or foam i ' m not judging at some point in the past 78 years was probably about right because our iron had more unintended defects which might have caused oxidation to occur more rapidly which would further impact spring performance because as metallurgists are soooooo willing to point out to me ceramics in this case iron oxide are brittle . Now i ' m not entirely sure about memory foam but i imagine there is a certain limit before it can no longer maintain its original ability to support so the back soooooo i ' m just ignoring that and water beds . TlDR in the past the statement probably held true in the present ehhhhh its too much math with too little available data but it might be closer to 1011 years before you should switch out a matress for back support .