Javascript Age Validation

It will display an alert message if your age is eligible to register into this website or not

function getAge(dateString) {
    var today = new Date();
    var birthDate = new Date(dateString);
    var age = today.getFullYear() - birthDate.getFullYear();
    var m = today.getMonth() - birthDate.getMonth();
    if (m < 0 || (m === 0 && today.getDate() < birthDate.getDate())) {
        age--;
    }
    return age;
}
if(getAge("1991/07/04")>20){
 alert("Your are eligible");
}else alert("Your are not eligible");