Bill Erickson

When editing a post, check “Mark as Old” in the metabox labeled “Old Post Notification” to add the notification to the top of the post. You can also define a custom message for this specific post. If no custom message is provided, the default message set in Settings > Old Post Notification will be used.
Further Customization with filters, described in documentation.
<a href="https://github.com/billerickson/old-post-notification/wiki" rel="nofollow ugc">Documentation</a> | <a href="https://github.com/billerickson/old-post-notification/issues" rel="nofollow ugc">Support Forum</a>

Old Post Notification

Mark posts as old and display a notification message above the content.

Chris Reynolds

The purpose of this plugin is to shame content editors into updating page content that has not been touched in a long time, while simultaneously providing an overview of the oldest pages on a site. Also: gifs.
The plugin adds an Out of Date Pages subpage under the Pages menu, which breaks all the pages on the site down by last modified date, grouped by years since it’s been modified. Each page listed has a link to edit the page. All images on the Out of Date Pages subpage are sourced from <a href="http://reactiongifs.com" rel="nofollow ugc">reactiongifs.com</a>.

Update Shaming

The only WordPress plugin not afraid to give you dirty looks.

Old Post Notification vs Update Shaming