unified age check vars to just have is_minor.

assets/js/form.js

@@ -354,7 +354,6 @@
 
   function addComputedAgeFlags(data) {
     const adultFlag = deriveAdultFlagFromBirthdate(data.geburtsdatum || '');
-    data.is_adult = adultFlag;
     data.is_minor = adultFlag === '' ? '' : adultFlag === '1' ? '0' : '1';
     return data;
   }

src/Form/Validator.php

@@ -132,13 +132,11 @@ final class Validator
         $adultFlag = $this->deriveAdultFlag((string) ($data['geburtsdatum'] ?? ''));
         if ($adultFlag === null) {
             return [
-                'is_adult' => '',
                 'is_minor' => '',
             ];
         }
 
         return [
-            'is_adult' => $adultFlag ? '1' : '0',
             'is_minor' => $adultFlag ? '0' : '1',
         ];
     }