You can set a chron task - doexpire.php - and it will delete threads over a certain age. I'm not sure if it goes by last replied to or if it goes by creation date. I don't delete threads because they are old. Once its up its up. So I don't know the specifics. My computer with my files is out of town right now so I can't look. You might want to read throught the doexpire.php. Don't change anything without having a back up copy though. The files pretty document what they are doing pretty well so you might read through it and decide that is what you need.
