title: mongodb how to get max value from collections
body: <p>I have a mongodb collection like:</p>
  
  <pre><code>db.kids.find()
  //results
  [
      {name:'tom', age:10},
      {name:'alice', age:12},
      ....
  ]
  </code></pre>
  
  <p>I need a query to get MAX 'age' from this collection
  like in SQL: <code>SELECT MAX(age) FROM kids WHERE 1</code></p>