Users that own latest version of MS Access can resave newer .accdb files to previously used .mdb files with no trouble. The problem with accdb to mdb conversion is usually encountered by users that only have old version of the program. You can find several 3rd party alternatives designed for MS Access database conversions, just keep in mind that not all features from latest ACCDB might be exported to old format.
Open ACCDB file Open MDB fileManagement tool for Access databases on Mac
A powerful database to database converter