Mark posts as old and display a notification message above the content.
Old Post Notification Download Now
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Originals Link WordPress.org
How to Install Old Post Notification Plugin, Manually
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.
Plugin Not Working?
If any errors or problems with the plug-in then write comments.