Custom field will do the task.
On the post page there’s nothing to do great, just create a Custom Field called agerating or age-rating. Authors will enter the value of the field according to their content with G/PG/R/X.
Then at the frontend, edit single.php. Use this code inside the loop to retrieve the rating value:
<?php $age-rating = get_post_meta($post->ID, 'age-rating', true); ?>
Now use this code to show the rating:
Rated As: <?php echo $age-rating; ?>
Change the codes according to your needs. You can also style the Custom value box inside the Post panel after becoming experienced with this.
More Details: http://codex.wordpress.org/Custom_Fields