Perhaps it's a sign of not getting the kind of heavy-hitting traffic you apparently are getting on a daily basis, but I see no need to close comments on an arbitrary date-based basis, spam be damned.

There are only a handful — less than five — of posts for which comments have been closed, and I only closed them because they were generating nothing BUT spam comments. As a general rule, however, I prefer to leave comments open, because you never know when a new spark of conversation left by someone who had the misfortune of not viewing your post within a 90-day window of its appearance might prompt a new addition to your online community, not to mention a topic for another new post.

There's something unfriendly to me about systematically closing down comments on a blog.