Age calculate by Birthday

if(crmForm.all.birthdate.DataValue != null)
{
    var now = new Date();
    var birthday = crmForm.all.birthdate.DataValue;
    var monthdif = now.getMonth() - birthday.getMonth();
    if(monthdif > -1)
        crmForm.all.new_age.DataValue = now.getYear() - birthday.getYear();
    else
        crmForm.all.new_age.DataValue = now.getYear() - birthday.getYear() - 1;
}