We keep the content because, no matter what the age of the content, it can still be useful to those seeking answers. Many of our members find MedHelp via google, which sometimes leads to these old discussions.
While it isn't ideal that these old threads get bumped to the top by new posts from time to time, we want to offer as many ways as possible for new members to find us and get help. If you see this happening, you are welcome to suggest that the new member start his or her own thread, where their question will get more attention.