Tag support for WordPress pages

functions.php

<?php
function my_theme_init() {
	register_taxonomy_for_object_type( 'post_tag', 'page' );
}

add_action( 'init', 'my_theme_init' );

function my_theme_tags_support_query( $wp_query ) {
	if ( $wp_query->get( 'tag' ) ) {
		$wp_query->set( 'post_type', 'any' );
	}
}

add_action( 'pre_get_posts', 'my_theme_tags_support_query' );