Old mattresses have lost some of their firmness, tend not to support your curves well, and lack some of the newer technology. Your old mattress might be causing or adding to your low back pain.
Do look for a mattress that is comfortable and supportive. One that keeps the spine aligned will relieve pressure points and help prevent or alleviate back pain, according to SleepHelp.org. Medium firmness mattresses with a top layer of foam that is at least three inches thick are often best. Some people with back pain use adjustable frames, where you can lower or raise your head and feet to help reduce back pain.