BackboneJS-Model validationError

BackboneJS-Model validationError

<!DOCTYPE html>
   <head>
      <title>Model Example</title>
         <script src="https://code.jquery.com/jquery-2.1.3.min.js" type="text/javascript"></script>
         <script src="https://cdnjs.cloudflare.com/ajax/libs/underscore.js/1.8.2/underscore-min.js" type="text/javascript"></script>
         <script src="https://cdnjs.cloudflare.com/ajax/libs/backbone.js/1.1.2/backbone-min.js" type="text/javascript"></script>
   </head>
   <body>
   <script type="text/javascript">
      var Person = Backbone.Model.extend({
         defaults: {
            name: 'john',
            age: 25,
            occupation: 'working'
         },
         initialize : function(){
            this.on("invalid",function(model,error){
            document.write(error);
         });
         },
         validate: function(attributes) {
            if ( attributes.age < 25 ) {
               return 'please enter the correct age!!! ';
            }
           if ( ! attributes.name ) {
              return 'please enter the name!!!';
           }
         },
   });
   var person = new Person();
   person.on('invalid', function() {
      this.arguments;
   });
   person.set({ age : '20' }, { validate : true });
   </script>
   </body>
</html>