How to Add a checkout birth date required field with adult control in Woocommerce

The following code adds a billing birth date field and will check customer age avoiding checkout if customer is not at least 18 year old:

// Adding a custom checkout date field
add_filter( 'woocommerce_billing_fields', 'add_birth_date_billing_field', 20, 1 );
function add_birth_date_billing_field($billing_fields) {

    $billing_fields['billing_birth_date'] = array(
        'type'        => 'date',
        'label'       => __('Birth date'),
        'class'       => array('form-row-wide'),
        'priority'    => 25,
        'required'    => true,
        'clear'       => true,
    );
    return $billing_fields;
}


// Check customer age
add_action('woocommerce_checkout_process', 'check_birth_date');
function check_birth_date() {
    // Check billing city 2 field
    if( isset($_POST['billing_birth_date']) && ! empty($_POST['billing_birth_date']) ){
        // Get customer age from birthdate
        $age = date_diff(date_create($_POST['billing_birth_date']), date_create('now'))->y;

        // Checking age and display an error notice avoiding checkout (and emptying cart)
        if( $age < 18 ){
            wc_add_notice( __( "You need at least to be 18 years old, to be able to checkout." ), "error" );

            WC()->cart->empty_cart(); // <== Empty cart (optional)
        }
    }
}

Code goes in the functions.php file of your active child theme (or active theme). Tested and works.