title: Python pandas groupby aggregation
body: <p>I have a <code>DataFrame df</code>, composed of <code>(age, height)</code>. I want to see how the mean of height changes with age, so I group <code>df</code> by <code>age</code> and try to form a new <code>DataFrame new_df</code>, composed of <code>(age, mean_height)</code>, code goes below:</p>
  
  <pre><code>groups = df.groupby('age')
  new_df = groups.agg({'height' : np.mean,
                       'age' : # HOW to add age?})
  </code></pre>
  
  <p>but I don't know how to append <code>age</code> to <code>new_df</code>, hope anyone could give me some advice.</p>