There's already a per-blog setting for it, at least in "Phoenix".

And you can achieve the same "quite" easily with simple SQL, which of course is not the user-friendly way..

UPDATE evo_posts
   SET post_comments = "closed"
 WHERE 1

instead of "WHERE 1" you could also limit it to old posts:

UPDATE evo_posts
   SET post_comments = "closed"
 WHERE post_issue_date < "2006-01-01"

would close them for items/posts with an issue date before the 1th Jan 2006.

A while back I made up a hack that automagically closes comments after a certain number of days that the user gets to set. I'm not sure if it's in the forums but [url=http://wonderwinds.com/hackblog.php/2004/11/02/closing_the_door_on_comment_spammers]it's on my hackblog[/url]. Sort of automates what blueyed showed, only you still have to hack it in there. Eventually we'll be able to make stuff like this be a plugin, but for now hacks are the best tool available.

Oh and I don't know what version was current when I wrote that so I don't know if it's directly workable with version .9.1, and I really doubt it'll be happy with version 1.6. Lotsa changes from the .9 generation to the phoeniz generation.