The Content Audit plugin has been updated to version 1.2. It now has options to automatically mark things outdated after a configurable time period — 3 months, a year, whatever. You can then email the assigned content owner or the original author a list of all their outdated content, again on a configurable schedule.
I recommend that you try the email notifications on a test installation first, with yourself as the owner of all the content. See the beta test notes for instructions on changing the owner of all the content in your test database.
You can grab 1.2 from the plugin repository.