feat: unique email addresses of participants unless child, cleanup

addresses #869axbn21

src/Form/Model/ParticipantDto.php

@@ -224,4 +224,26 @@ class ParticipantDto
             'message' => $message,
         ];
     }
+
+    /**
+     * Determines if the participant is a child based on age at current date.
+     *
+     * A child is defined as someone under the specified age threshold (default: 16 years).
+     * This classification is used for email uniqueness validation (children can share emails with adults).
+     *
+     * @param int $ageThreshold The age threshold for child classification (default: 16)
+     *
+     * @return bool True if participant is under the age threshold, false otherwise or if age unknown
+     */
+    public function isChild(int $ageThreshold = 16): bool
+    {
+        $age = $this->getAge();
+
+        // Treat unknown age as adult for safety (requires email uniqueness)
+        if (null === $age) {
+            return false;
+        }
+
+        return $ageThreshold > $age;
+    }
 }