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.

jonkern

Keep it Fresh will stop WordPress from showing posts older than a set number of days or a specific date. It is configurable via the WordPress administration area. Keep it Fresh doesn’t remove old posts, nor does it muck with their <code>post_status</code>; rather, Keep it Fresh uses the <code>posts_where</code> WordPress filter to filter out posts older than the given date or amount of days whenever WordPress requests posts outside of the admin area.

Keep it Fresh

Keep it Fresh will stop WordPress from showing posts older than a set number of days or a specific date.

Old Post Notification vs Keep it Fresh