hi all i have added 2 fields in my odoo hr module , while updating the DOB it should return me the age in age filed but its not retuning someone help me with this , iam using odoo13

hr.py:

    import base64

from random import choice

from string import digits

import itertools

from werkzeug import url_encode

import pytz

from datetime import datetime

import datetime

from odoo import api, fields, models, _

from odoo.exceptions import ValidationError, AccessError

from odoo.modules.module import get_module_resource

from odoo.addons.resource.models.resource_mixin import timezone_datetime

from datetime import *

from datetime import date 

from datetime import datetime

from dateutil.relativedelta import relativedelta

birthday = fields.Date(string="DOB")

age = fields.Integer(string="Age")

@api.onchange('birthday')

def _onchange_birth_date(self):

    """Updates age field when birth_date is changed"""

    if self.birthday:

        d1 = self.birthday

        d2 = date.today()

        self.age = relativedelta(d2, d1).years