Javascript: Only submit answers if one of the checkboxes is checked

An alert box is shown if no check box was checked when submitting an answer. The idea is that at least one check box should be checked in each question. Below is the code:

<form method="post" action="take_test.php" name="test_question" onsubmit="return checkForBlanks();">
Are you? <br />
<input type="radio" name="age" value="old" />Old<br />
<input type="radio" name="age" value="middle" />Middle Age<br />
<input type="radio" name="age" value="young" /> Young
</form>
var the_question  = document.forms['test_question'].elements['age'];
function checkForBlanks(){
        var blank_entry = true;
        for(var i=0; i<the_question.length; i++){
              
                         if(the_question[i].checked){
                
                            blank_entry = false;
                
                             break;
   
                          }
           
            }
        if(blank_entry){
     
              alert("You do not  answer  this question. Please answer it.");
   
                 return false;
         }
        else{
              return true;
        }

}