Add checkbox to WooCommerce checkout process

wc_add_checkbox_to_checkout.php

<?php
    // This snippet will add a checkbox that a user needs to check before checkout. 
    // Triggers if a certain product belongs to a 'restricted' categroy.

    add_action( 'woocommerce_review_order_before_submit', 'add_terms_and_conditions_checkbox' );

    function add_terms_and_conditions_checkbox() {
        $terms = array(
            'restricted' => '<strong>' . __("I confirm I'm X years old", 'text-domain') . '</strong>', //Set checkbox text here
        );

        $displayed = array(); // keep track of which checkboxes have been displayed

        // check if cart contains products from specified categories
        foreach ( WC()->cart->get_cart() as $cart_item ) {
            $product_id = $cart_item['product_id'];
            $product_categories = wp_get_post_terms( $product_id, 'product_cat', array( 'fields' => 'slugs' ) );

            foreach ( $terms as $category => $message ) {
                if ( $category === 'restricted' && in_array( $category, $product_categories ) && !in_array( $category, $displayed ) ) {
                    // add terms and conditions checkbox
                    woocommerce_form_field( 'terms_'.$category, array(
                        'type'          => 'checkbox',
                        'class'         => array('form-row terms'),
                        'label_class'   => array('woocommerce-form__label woocommerce-form__label-for-checkbox checkbox'),
                        'input_class'   => array('woocommerce-form__input woocommerce-form__input-checkbox'),
                        'required'      => true,
                        'label'         => $message,
                    ));
                    $displayed[] = $category;
                }
            }
        }
    }

    // Validate terms and conditions checkboxes
    add_action( 'woocommerce_checkout_process', 'validate_terms_and_conditions_checkbox' );

    function validate_terms_and_conditions_checkbox() {
        $terms = array(
            'restricted' => __("Please accept the terms and conditions for Online Courses", 'text-domain'), //Set error notice here
        );

        foreach ( WC()->cart->get_cart() as $cart_item ) {
            $product_id = $cart_item['product_id'];
            $product_categories = wp_get_post_terms( $product_id, 'product_cat', array( 'fields' => 'slugs' ) );

            foreach ( $terms as $category => $error_message ) {
                if ( $category === 'restricted' && in_array( $category, $product_categories ) && ! isset( $_POST['terms_'.$category] ) ) {
                    wc_add_notice( $error_message, 'error' );
                }
            }
        }
    }
?>