I do not believe that old notes should be pruned. As it is, they serve as a good refrence for older topics and, being digital, there is no real storage advantage to disposing of them. Also, those who read and reply to threads should take note of the date that messages were posted; this should not be a major issue to most.
The amount of space used by this forum is so tiny its not even worth considering on the basis of saving "space".. The entire forum is barely 10 megabytes.
Keeping posts allows for a history and archive, unlike the noteboards within the game which are constantly cleaned, meaning people miss announcement or information.
There's no harm in keeping threads around, I'm assuming everyone can look at a date and decide if something is really old or not.