This plugin adds an age check to the checkout form. Customers have to tick a box in order to be able to place an order. 
You can customise the messages and enable/disable the function from the Woocommerce settings page. 
Works like a charm with WordPresss 5.3 and the latest versions of Woocommerce.

Age Checkbox for-Woocommerce

This plugin adds a customisable age check to the checkout form. Customers have to tick a box in order to be able to place an order.

cookers DEVELOPMENT

This plugin provides an overlay for each website user to confirm his age (or different things – the text is fully costomizable in the options). On right answer, a cookie is set to remember the user for a set amount of days and not show the nag screen again. Otherwise, a custom message is shown to the user and the access to the site is not granted.
The plugin was initially developed for the age restriction of selling alcoholic beverages in a webshop. Due to legal reasons, such an age restriction must be in place (depending of the country of operation).

Quick Age Verification

Presents each site user a quick age verification screen before using the site. Saves the response in a cookie for further visits.

Age Checkbox for-Woocommerce vs Quick Age Verification