So long as the database and/or forum operation is not hampered by the size of the database, it would, IMHO, be a mistake to drop "old" threads. One of the nice features about any well-written forum script is the ability to do some fairly comprehensive searches within the forum contents. This gives all forum readers the ability to find possible answers to their current questions that have already been posted in response to another reader's question, if they take the time to do a quick search. Further, it also gives those posting responses the ability to point a reader to previously posted responses when appropriate.
For example, there are often questions asked that have been asked already, and sometimes, an excellent response exists as well. Why re-write the response if we can point the reader to the existing one instead?
Consider the time spent by some of the forum regulars in answering questions. Any time not spent in repetitive responses to common questions is time that can then be spent in helping other readers with new questions...
The forum history constitutes a certain body of knowledge. Would you discard reference books from your personal library just because they are "old"? I wouldn't! I sometimes have the need to look up information on a topic that I may not have dealt with in the past ten years, but at least I have the source material readily available when I need it. It's the same, albeit on a different scale, with the forum.