calculate age from javascript

function get_age(ymd) {  
    var x = ymd.split('-');
    var y = x[0];
    var m = x[1];
    var d = x[2];
    var today = new Date();
    var test = m = (today.getMonth()+1 - m);
    if (test < 0) {
        y++;
    } else if (m == 0 && today.getDate() - d < 0) {
        y++;
    }
    var test2 = today.getFullYear() - y;
    if (test2 < 0)
        test2 = 0;
    return test2;
}


alert(get_age('1980-01-01'));