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.
old_post_notification_post_types, post types this plugin is available to (default = 'post')
old_post_notification_on_content, turn off the display on content, useful if you want to position it yourself
old_post_notification_css, turn off the enqueued CSS, useful if you want to design the notification box yourself