title: Validation for an additional checkbox field in WooCommerce checkout page
body: <p>I have added in checkout page a acceptance field below the terms and conditions field. The new field is about the age validation.</p>
  <p>In frontend, I can see my checkbox but customer should not be able to order if checking this checkbox in not checked.
  This is the generated html code:</p>
  <pre><code>&lt;?php if ( wc_terms_and_conditions_checkbox_enabled() ) : ?&gt;
              &lt;p class=&quot;form-row validate-required&quot;&gt;
                  &lt;label class=&quot;woocommerce-form__label woocommerce-form__label-for-checkbox checkbox&quot;&gt;
                  &lt;input type=&quot;checkbox&quot; class=&quot;woocommerce-form__input woocommerce-form__input-checkbox input-checkbox&quot; name=&quot;terms&quot; &lt;?php checked( apply_filters( 'woocommerce_terms_is_checked_default', isset( $_POST['terms'] ) ), true ); // WPCS: input var ok, csrf ok. ?&gt; id=&quot;terms&quot; /&gt;
                      &lt;span class=&quot;woocommerce-terms-and-conditions-checkbox-text&quot;&gt;&lt;?php wc_terms_and_conditions_checkbox_text(); ?&gt;&lt;/span&gt;&amp;nbsp;&lt;span class=&quot;required&quot;&gt;*&lt;/span&gt;
                  &lt;/label&gt;
                  &lt;input type=&quot;hidden&quot; name=&quot;terms-field&quot; value=&quot;1&quot; /&gt;
              &lt;/p&gt;
          &lt;?php endif; ?&gt;
                          &lt;p class=&quot;form-row validate-required&quot;&gt;
                  &lt;label class=&quot;woocommerce-form__label woocommerce-form__label-for-checkbox checkbox&quot;&gt;
                  &lt;input type=&quot;checkbox&quot; class=&quot;woocommerce-form__input woocommerce-form__input-checkbox input-checkbox&quot; required name=&quot;validationage&quot;  id=&quot;validationage&quot; /&gt;
                      &lt;span class=&quot;woocommerce-terms-and-conditions-checkbox-text&quot;&gt;&lt;?php _e( 'Je certifie avoir + de 18 ans', 'woocommerce'); ?&gt;&lt;/span&gt;&amp;nbsp;&lt;span class=&quot;required&quot;&gt;*&lt;/span&gt;
                  &lt;/label&gt;
              &lt;/p&gt;
  </code></pre>
  <p>How to enable the validation for this checkbox?</p>