kco-validate-customer-age.php

<?php
// 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();
	}
}