How to Calculate Age using javaScript

function CalculateAge(birthday)
{


    var re = /^(0[1-9]|1[012])[- /.](0[1-9]|[12][0-9]|3[01])[- /.](19|20)\d\d+$/;

    if (birthday.value != '')
    {

        if (re.test(birthday.value))
        {
            birthdayDate = new Date(birthday.value);
            dateNow = new Date();

            var years = dateNow.getFullYear() - birthdayDate.getFullYear();
            var months = dateNow.getMonth() - birthdayDate.getMonth();
            var days = dateNow.getDate() - birthdayDate.getDate();
            if (isNaN(years)) {

                document.getElementById('lblAge').innerHTML = ''
                ";
document.getElementById('lblError').innerHTML = 'Input date is incorrect!';
return false;

}

else
{
document.getElementById('lblError').innerHTML = ''";
                document.getElementById('lblAge').innerHTML = years + ' Years ' + months + ' months ' + days + ' days';

            }
        }
        else
        {
            document.getElementById('lblError').innerHTML = 'Date must be mm/dd/yyyy format';
            return false;
        }
    }
}