Code example on how to only accept purchases from people over 18 years of age with Klarna Checkout and WooCommerce

wc-kco-validate-customer-age.php

<?php
/**
 * WooCommerce - Klarna payment gateway
 * Only accept purchases from customer over 18 years of age.
 **/

// Edit the order data sent to Klarna to make 'Date of birth' a mandatory field in the KCO iframe
add_filter('kco_create_order', 'my_kco_create_order');
function my_kco_create_order( $create ) {
	$create['options']['national_identification_number_mandatory'] = true;
	return $create;
}

// Validate the customers age on the validate callback from Klarna (happens right before the purchase is finalized).
// The 'Check items stock and valid shipping method during checkout' setting needs to be checked in the KCO settings for this to work.
// The store needs to have SSL/https enabled for the validate callback to work.
add_action('kco_validate_checkout', 'my_kco_validate_checkout' );
function my_kco_validate_checkout( $data ) {
	// $data is the returned order data sent from Klarna
	$birthday = new DateTime($data['customer']['date_of_birth']);
	$now = new DateTime();
	$age = $now->diff($birthday);
	if( $age -> y < 18 ) {
		header( 'Location: ' . wc_get_checkout_url() . '?under_age' );
		exit();
	}
}

// Display an error notice abouve the KCO checkout if the customer is under 18 years.
add_action('klarna_before_kco_checkout', 'my_klarna_before_kco_checkout' );
function my_klarna_before_kco_checkout() {
 	if ( isset( $_GET['under_age'] ) ) {
		echo '<div class="woocommerce-error">';
		_e( 'You need to be over 18 years to make a purchase.', 'woocommerce-gateway-klarna' );
		echo '</div>';
	}
}

https://gist.github.com/mattiasghodsian/a84d98d4f89eb57234d9a28e01b16865