Typically in a Social Security disability claim, DDS only obtains the medical records for one year prior to your alleged onset date. DDS is concerned with the severity of your impairments during the period of disability. They understandably focus their efforts on that time frame.
However, older medical records can be important. First, older record can help to establish a favorable onset date. And if there is a previous application, you are going to want to get the older records for reopening.
More importantly for me, the older records can help to tell the story. Every case has a story, and older medical records provide invaluable context for the judge at the hearing. Often, for example, a claimant has been struggling with an impairment for years, and only recently did the impairment become truly disabling. Those older records will help to the judge to see what changed to make employment impossible. So get the older records.
Hat tip to Tim Moore, who mentioned obtaining older records in a recent article discussing tips for your disability claim.