It really depends on the content of your site. If you're truly writing timeless pieces (i.e. tutorials), then please leave comments open. The content is relevant today, 180 days from now, and maybe even 2 years from now. If you're curating your content and keeping it up-to-date, leaving comments open on old posts is even more important.

But if you're running a news site and the content (and conversation) is outdated after a period of time, there's no harm in closing comments.

In the end, there's no hard-and-fast rule that will fit for all sites.