只需查询最新帖子并比较ID：

  function wpse110867_is_latest_post( $post_id, $query_args = array() ){
      static $latest_post_id = false;
      $post_id = empty( $post_id  ) ? get_the_ID() : $post_id ;

      if( !$latest_post_id ){
          $query_args[\'numberposts\'] = 1;
          $query_args[\'post_status\'] = \'publish\';
          $last = wp_get_recent_posts( $query_args );
          $latest_post_id = $last[\'0\'][\'ID\'];
      }

      return $latest_post_id == $post_id;
  }

if( wpse110867_is_latest_post() ){
   echo \'This is the latest post\';
}

if( wpse110867_is_latest_post( get_the_ID(), array( \'post_type\' => \'my-post-type\' ) ){
   echo \'This is the latest post\';
}