how to alert when user enters nothing : i'm making a javascript and i want to alert when the user enters nothing in the prompt  

<!-- begin snippet: js hide: false console: true babel: false -->

<!-- language: lang-js -->

    function verifyage() {
    var age = prompt("enter your age:", "");
    var status = (age >= 18) ? "adult" : "minor";
    if(status == "minor") {
    alert("you are too young");
    } else if (status == "adult") {
    alert("you're in");
    } else if (status == "null") {
    alert("you need to enter your age");
    }
    else {
    void(0);
    }}

<!-- language: lang-html -->

    <button onclick="verifyage();" type="button">watch movie</button>

<!-- end snippet -->

what it's supposed to do:  
define a variable age which defines the age of user  
define a variable status which has two value adult and minor  
prompt for the age of the user  
alert the message you are too young when the user enter a number less than 18  
alert the message you're in when user enters number greater than 18  
alert the message you have to enter your age when user enters nothing  
but when i test it and enter nothing it says you are too young why?  
can you help me?  
i tried http://stackoverflow.com/questions/10577549/how-to-show-alert-when-user-enter-other-then-number-value-in-textfield and other but nothing  
plz help me