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.

boxuk

OneID

The official WordPress Plugin for OneID® from OneID Limited. Allows WooCommerce store owners to verify the age of their customers before purchase.
<h3>Key Features</h3>
Free: OneID is free to use! 
Painless Integration: Download the plugin, register, and off you go! 
Only Share Necessary Data: Customers only share the data that is required.  
Legally Compliant: Meet all current regulations for your products and services. 
Complete Market Coverage: Age check anyone with an online UK bank account. 
Hassle Free: No document uploads, live selfies, or manual checks are required. 
<h3>Signing Up</h3>
You can try the product first try it free <a href="https://www.oneid.uk/woocommerce-age-verification#form" rel="nofollow ugc">here</a> to register for Sandbox-only access.
Once you have completed trialing and are ready to progress, complete <a href="https://www.oneid.uk/woocommerce-age-verification#form" rel="nofollow ugc">this form</a> to register for Production access.
<h3>About OneID® Age Verification</h3>
Be confident you’re meeting legal requirements with OneID®, a fully regulated online identity and age verification service. 
Our fast, simple, and secure identity checks let you verify the age of your customers in real-time before they’ve placed their order – saving you time, hassle, and security risk.  
<h3>How does it work?</h3>
Whenever you need to verify the age of your customers, OneID® can help. 
Instead of relying on customers to check a box, enter their birthday, or go through a lengthy document upload process, OneID® lets your customers verify themselves in real-time, before their purchase, in a matter of seconds.  
Customers verify they are eligible to buy age-restricted goods via the information held at their bank. You can rest easy knowing your customers are protected, your business is protected, and your customer experience remains hassle-free! 
Our age verification checks allow you to easily conduct regulation-compliant age verification in seconds for WooCommerce stores selling age-restricted products such as:  
<ul>
<li>
Alcohol
</li>
<li>
E-Cigarettes and Vapes
</li>
<li>
Kitchenware (Knives)
</li>
<li>
Pharmaceuticals
</li>
<li>
Fireworks
</li>
<li>
Pets and live animals
</li>
<li>
DVDs and Games
</li>
<li>
Home improvement and DIY tools (knives, tools, corrosive, or noxious substances)
</li>
<li>
Gardening (tools, sharp objects, noxious substances, and some seeds)
</li>
<li>
And any other products/ services you’d like to restrict access to.
</li>
</ul>
<h3>Pricing Plan</h3>
<ul>
<li>
No monthly subscription charges.
</li>
<li>
Free age verification checks.
</li>
</ul>

OneID®  – Age Verification Identity Solution

The official WordPress Plugin for OneID® from OneID Limited. Allows WooCommerce store owners to verify the age of their customers before purchase.

Age Checkbox for-Woocommerce vs OneID® – Age Verification Identity Solution