Append new Item to a list in Vue 2

I have a very simple form which gets name and age from user and add this to a list.

You can check that on JSFiddle.

Here is HTML :

<div id="app">
<ul>
  <li v-for="user in users">{{ user.name +' '+ user.age }}</li>
</ul>
<hr>
  <form v-on:submit.prevent="addNewUser">
    <input v-model="user.name" />
    <input v-model="user.age" />
    <button type="submit">Add User</button>
  </form>
</div>

Here is Vue code:

new Vue({
   el: '#app',
   data: {
    users: [{name:"Mack", age:20}],
    user: {name:"", age:0}
   },
   methods: {
     addNewUser() {
        this.users.push(this.user);
     }
   }
});

The Problem

The problem is where I trying to add more than one user to the list. As you can see, when I type new value for the new user, previous values of recently added users change too!

How can I fix it?