<?php 
function close_comments( $posts ) {
	if ( !is_single() ) { return $posts; }
	if ( time() - strtotime( $posts[0]->post_date_gmt ) > ( 30 * 24 * 60 * 60 ) ) {
		$posts[0]->comment_status = 'closed';
		$posts[0]->ping_status    = 'closed';
	}
	return $posts;
}
add_filter( 'the_posts', 'close_comments' ); 
?>