Here is the code;

To use this code open your functions.php and paste this code into end of file.

function is_old_post($post_id=null){
   $days = 15;
   global $wp_query;
   if(is_single() || is_page()) {
      if(!$post_id) {
         $post_id = $wp_query->post->ID;
      }
      $current_date = time();
      $offset = $days *60*60*24;
      $post_id = get_post($post_id);
      $post_date = mysql2date('U',$post_id->post_date);
      $cunning_math = $post_date + $offset;
      $test = $current_date - $cunning_math;
      if($test > 0){
         $return = true;
      }else{
         $return = false;
      }
   }else{
      $return = false;
   }
   return $return;
}

After that use this code for show your ads…

<?php if(is_old_post()){ ?>
INSERT AD CODE HERE
<?php } ?>

This function show ads on your 15 days old posts. To change this simply change $days value