Add field validation to WooCommerce Date-picker on checkout form

We are using WooCommerce Checkout Field Editor Pro to create a date of birth age check on our checkout page. We require this page to return an error "You must be over 18 to purchase products from Hopbox" if the date of birth selected by the customer is less than 18 years old. We are currently using another plugin that has the correct function, but it does no't allow the customisation we require. It might be possible to extract the field validation code from this plugin. Otherwise, this website has some validation code that would work: [login to view URL] It would just need to be implemented with the correct tags.