Hello guys, please help me to do validation in my custom module.

i want to develop a HR module with Employee name, date of birth and age. in my module i want validate age should above 18 years and do not exceed 120 years.

please let me know, how to improve my code here is my code.

in my .py file

from osv import fields, osv
from datetime import datetime
from time import strptime
from dateutil import parser

class employee_employee(osv.osv):

    _name='employee.employee'

#   Age calculation

    def onchange_age(self, cr, uid, ids, DOB, context=None):
        current_date=datetime.now()
        current_year=current_date.year
        birth_date = parser.parse(DOB)

        if birth_date.year > current_year-120 and birth_date.year<current_year-18:
            current_age=current_year-birth_date.year
        else:
            raise osv.except_osv(_('Invalid DOB'), _('Please enter a valid DATE OF BIRTH'))

        val = {
        'age':current_age
        }
        return {'value': val}

    _columns ={ 
          'name':fields.char('Emp Name',size=64, required=True),
          'DOB':fields.date('DOB'),
                  'age':fields.char('Age'),
        }

employee_employee()

in my .xml

   <record id="employee_form_view" model="ir.ui.view">
        <field name="name">employee.employee.form</field>
        <field name="model">employee.employee</field>
        <field name="type">form</field>
        <field name="arch" type="xml">

           <form string="employee">
            <field name="name"/>
                    <field name="DOB" />
                    <field name="age"/>
            </form>
        </field>
    </record>

Thanks in advance....