I only saw 3 threads that went back further than this year when I looked (they've now been split by year), but I see where you're coming from.  I'd say that due to the nature of message boards, popular topics are going to linger.  I generally try to trim topics by season to keep them from getting too unwieldy, but sometimes I miss a few.

Do you know how to use the "first new post" and "most recent post" buttons for topics?  I'd be happy to slap together a quick tutorial if not.  There's also an option in profile settings that will allow you to arrange threads in reverse chronological order (with the newest post always at the top of the first page).  I'm not sure if that would be more to your liking or not, but there are some people who use that option.