You have 2 options here. Either enable moderation (no comments are auto-published) or add some PHP to check if the item was created more than 10 days ago and then hide the form entirely.
At the very top of your item.php override and after "defined('_JEXEC') or die;", add this inside the <?php ... ?> block:
$itemDate = strtotime($this->item->created);
$now = strtotime('now');
$diff = number_format(($now - $itemDate)/(3600*24));
"$diff" returns the difference in days between now and the time the item was created. So if you want to hide the comments form 10 days after an item was created, update the 2 instances of "<?php echo $this->loadTemplate('comments_form'); ?>" with:
<?php if ($diff <= 10): ?>
<?php echo $this->loadTemplate('comments_form'); ?>
<?php endif; ?>
