Keep on getting "Bummer! A young player was accepted as valid!". Where is the mistake

from rest_framework import serializers

from . import models


class GameSerializer(serializers.ModelSerializer):
    class Meta:
        fields = ('id', 'name')
        model = models.Game


class PlayerSerializer(serializers.ModelSerializer):
    class Meta:
        fields = ('id', 'name', 'age')
        model = models.Player

    def validate_age(self, age):
        if age >= 13:
            return age
        raise serializer.ValidationError(
            "Player age must be 13 or over")

class ScoreSerializer(serializers.ModelSerializer):
    game = serializers.HyperlinkedRelatedField(
        read_only=True, view_name='apiv2:game-detail')
    player = serializers.HyperlinkedRelatedField(
        read_only=True, view_name='apiv2:player-detail')

    class Meta:
        fields = ('id', 'game', 'player', 'score')
        model = models.Score