Plugin to make posts older than x private?

I was wondering if there is a plugin that will make posts older than x days private (say, older than 30 days)? I am using a plugin that will close comments on any post older than x days, and am considering making most of my post archives private (for privacy reasons). I would prefer if there was a plugin, much like the comment plugin, that would do this automatically instead of my having to do this manually.
Thanks for any suggestions.