jquery check if checkbox is not checked

if (!$("#checkboxID").is(":checked")) {
    // do something if the checkbox is NOT checked
}

$('#isAgeSelected').click(function() {
    $("#txtAge").toggle(this.checked);
});
<script src="https://ajax.googleapis.com/ajax/libs/jquery/2.1.1/jquery.min.js">
  </script>
<input type="checkbox" id="isAgeSelected"/>
<div id="txtAge" style="display:none">Age is something</div>

 $('#checkAll').click(function () {    
     $('input:checkbox').prop('checked', this.checked);    
 });