There are some interesting behavioral nudges that community design can do better at.
If a user arrived at the old topic from 3 years ago from a search engine, he may not know (or know where to look!) that the topic is old. The software should probably give a gentle reminder that the topic might be out of date and ask if she is sure she wants to post.
I also think there's some pruning that admins or moderators tend to forget about. We talk about relevancy, but why do we trust our users to assess if it's relevant and timely? Admins should be archiving or classifying outdated information in an annual cleanup, so topics are clearly archived or moved to a board that clearly explain why the topics are no longer relevant.