Closes comments on old posts on the fly, without any extra DB queries. Hooked to the_posts.

function _close_comments_for_old_posts( $posts, $query ) {
	if ( empty( $posts ) || ! $query->is_singular() || ! get_option( 'close_comments_for_old_posts' ) ) {
		return $posts;
	}

	/**
	 * Filters the list of post types to automatically close comments for.
	 *
	 * @since 3.2.0
	 *
	 * @param string[] $post_types An array of post type names.
	 */
	$post_types = apply_filters( 'close_comments_for_post_types', array( 'post' ) );
	if ( ! in_array( $posts[0]->post_type, $post_types, true ) ) {
		return $posts;
	}

	$days_old = (int) get_option( 'close_comments_days_old' );
	if ( ! $days_old ) {
		return $posts;
	}

	if ( time() - strtotime( $posts[0]->post_date_gmt ) > ( $days_old * DAY_IN_SECONDS ) ) {
		$posts[0]->comment_status = 'closed';
		$posts[0]->ping_status    = 'closed';
	}

	return $posts;
}