title: Pandas find the mean of DataFrame based on a conditional across multiple columns
body: <p>I am trying to find the average weight of females under 20, I have the following DataFrame, I have already converted age to int and weight to float.</p>
  <pre><code> age   weight     height   male
   39.0   88.636360   180.0   True
   64.0   75.000000   155.0  False
   17.0  100.000000   183.0  False
   35.0   63.636364   170.0  True
   18.0   70.454544   173.0  False
  </code></pre>
  <p>I've tried <code>df.groupby(['male','age'])['weight'].mean()[False]</code> but it just returns something like:</p>
  <pre><code>age    
  18.0    64.225121
  19.0    65.499535
  20.0    67.855026
  21.0    69.622658
  22.0    69.376862
  </code></pre>
  <p>How can I filter it so that it aggregates the weight of all female under 20 then takes the average?</p>