Automatically disable comments for posts after X days?

I posted this to the Plugins and Hacks area with no results, so I’m thinking maybe it was the wrong place.

I’m looking for a way to automatically disable comments on posts after a certain number of days, in order to cut down on spam comments. I certainly don’t want to go back through my entire archive to manually disable them, much less do that every week or month or whatever.

I did search and browse through the plugins in the directory, the documentation, support forms, and so on but haven’t found anything that says it does what I need.

Thank you!