Do you have your comment spam filter set to automatically delete any comments on older posts? While this will get rid of a lot of spam for you it also could delete some real comments.
We have posts in our archives which we leave for anyone to find, through searching or through links on other sites. So it is very possible real commenters will come to an old post and leave a real comment. If you delete comments on old posts you won’t ever find any real comments there.
Unless you choose to delete your archived posts rather than keep them, shouldn’t you let people comment? Why would you close off the old posts and yet leave them online to be found? So many people argue that a blog isn’t even a blog unless it has comments, that interaction. Yet, they seem to do various things to shut down comments. It’s backwards – like a blog archive. Funny how it is all relative if you think about it.