diff --git a/Events.php b/Events.php
index 20d7bc4..0f1a3d7 100644
--- a/Events.php
+++ b/Events.php
@@ -283,6 +283,17 @@ public static function onAccountSettingsMenuInit($event)
         }
     }
 
+    public static function onBeforeValidate($event)
+    {
+        $registrationForm = $event->sender;
+        $minimumAge = Yii::$app->getModule('legal')->getMinimumAge();
+
+        if ($minimumAge > 0) {
+            $ageValidator = new validators\AgeValidator(['minimumAge' => $minimumAge]);
+            $ageValidator->validateAttribute($registrationForm, 'birthday');
+        }
+    }
+
     /**
      * Callback on daily cron job run
      */
diff --git a/config.php b/config.php
index f5fce73..09b2f3a 100644
--- a/config.php
+++ b/config.php
@@ -7,6 +7,7 @@
 use humhub\modules\content\widgets\richtext\ProsemirrorRichText;
 use humhub\modules\user\models\forms\Registration;
 use humhub\modules\user\widgets\AccountSettingsMenu;
+use humhub\modules\user\models\Profile;
 use humhub\widgets\FooterMenu;
 use humhub\widgets\LayoutAddons;
 
@@ -30,5 +31,6 @@
         ['class' => ProsemirrorRichText::class, 'event' => ProsemirrorRichText::EVENT_AFTER_RUN, 'callback' => ['humhub\modules\legal\Events', 'onAfterRunRichText']],
         ['class' => AccountSettingsMenu::class, 'event' => AccountSettingsMenu::EVENT_INIT, 'callback' => ['humhub\modules\legal\Events', 'onAccountSettingsMenuInit']],
         ['class' => CronController::class, 'event' => CronController::EVENT_ON_DAILY_RUN, 'callback' => ['humhub\modules\legal\Events', 'onCronDailyRun']],
+        ['class' => Profile::class, 'event' => Profile::EVENT_BEFORE_VALIDATE, 'callback' => ['humhub\modules\legal\Events', 'onBeforeValidate']],
     ],
 ];
diff --git a/docs/CHANGELOG.md b/docs/CHANGELOG.md
index b6baf78..834bfb3 100644
--- a/docs/CHANGELOG.md
+++ b/docs/CHANGELOG.md
@@ -4,6 +4,7 @@ Changelog
 1.4.3 (Unreleased)
 --------------------------
 - Fix #85: Fix downloading of large user export data file
+- Enh: Check User Birthday field
 - Enh #90: Use PHP CS Fixer
 
 1.4.2 (September 13, 2024)
diff --git a/tests/codeception/acceptance/LegalCest.php b/tests/codeception/acceptance/LegalCest.php
index 49c55e3..81a509c 100644
--- a/tests/codeception/acceptance/LegalCest.php
+++ b/tests/codeception/acceptance/LegalCest.php
@@ -136,4 +136,48 @@ public function testAgeVerification(AcceptanceTester $I)
         $I->amUser1(true);
         $I->dontSee($title, '.panel-heading');
     }
+
+    public function testAgeValidation(AcceptanceTester $I)
+    {
+        $I->wantTo('test age validation during registration and profile update');
+        $minimumAge = 18;
+
+        $I->amAdmin();
+        $I->amGoingTo('enable age verification');
+        $I->enableAgeVerification($minimumAge);
+
+        // Test registration with valid age
+        $I->amGoingTo('test registration with valid age');
+        $I->amOnRoute('/user/registration');
+        $I->fillField('Registration[username]', 'validAgeUser');
+        $I->fillField('Registration[email]', 'validage@example.com');
+        $I->fillField('Registration[password]', 'ValidPassword123');
+        $I->fillField('Registration[birthday]', date('Y-m-d', strtotime("-{$minimumAge} years -1 day")));
+        $I->click('Register');
+        $I->dontSee('You must be at least ' . $minimumAge . ' years old.');
+
+        // Test registration with invalid age
+        $I->amGoingTo('test registration with invalid age');
+        $I->amOnRoute('/user/registration');
+        $I->fillField('Registration[username]', 'invalidAgeUser');
+        $I->fillField('Registration[email]', 'invalidage@example.com');
+        $I->fillField('Registration[password]', 'InvalidPassword123');
+        $I->fillField('Registration[birthday]', date('Y-m-d', strtotime("-{$minimumAge} years +1 day")));
+        $I->click('Register');
+        $I->see('You must be at least ' . $minimumAge . ' years old.');
+
+        // Test profile update with invalid age
+        $I->amGoingTo('test profile update with invalid age');
+        $I->amUser1(true);
+        $I->amOnRoute('/user/account/edit');
+        $I->fillField('Profile[birthday]', date('Y-m-d', strtotime("-{$minimumAge} years +1 day")));
+        $I->click('Save');
+        $I->see('You must be at least ' . $minimumAge . ' years old.');
+
+        // Test profile update with valid age
+        $I->amGoingTo('test profile update with valid age');
+        $I->fillField('Profile[birthday]', date('Y-m-d', strtotime("-{$minimumAge} years -1 day")));
+        $I->click('Save');
+        $I->dontSee('You must be at least ' . $minimumAge . ' years old.');
+    }
 }
diff --git a/validators/AgeValidator.php b/validators/AgeValidator.php
new file mode 100644
index 0000000..2ea3f8a
--- /dev/null
+++ b/validators/AgeValidator.php
@@ -0,0 +1,57 @@
+<?php
+
+namespace humhub\modules\legal\validators;
+
+use DateTime;
+use Yii;
+use yii\validators\Validator;
+use humhub\modules\user\models\User;
+
+/**
+ * AgeValidator validates that the given value represents an age greater than or equal to a specified minimum age.
+ */
+class AgeValidator extends Validator
+{
+    /**
+     * @var int The minimum age required
+     */
+    public $minimumAge;
+
+    /**
+     * @inheritdoc
+     */
+    public function init()
+    {
+        parent::init();
+        if ($this->minimumAge === null) {
+            $this->minimumAge = Yii::$app->getModule('legal')->getMinimumAge();
+        }
+    }
+
+    /**
+     * Validates the age of the user based on the given attribute value.
+     *
+     * @param \yii\base\Model $model the data model being validated
+     * @param string $attribute the name of the attribute to be validated
+     */
+    public function validateAttribute($model, $attribute)
+    {
+        $value = $model->$attribute;
+        if (!$value instanceof DateTime) {
+            try {
+                $value = new DateTime($value);
+            } catch (\Exception $e) {
+                $this->addError($model, $attribute, Yii::t('LegalModule.base', 'Invalid date format.'));
+                return;
+            }
+        }
+
+        $today = new DateTime();
+        $age = $today->diff($value)->y;
+
+        if ($age < $this->minimumAge) {
+            $message = Yii::t('LegalModule.base', 'You must be at least {age} years old.', ['age' => $this->minimumAge]);
+            $this->addError($model, $attribute, $message);
+        }
+    }
+}