title: pandas/dataframe/datetimes how to extract age from brth_dt column
body: <p>I got a column “brth_dt” and its type was datetime64[ns],how to extract people's age from it?I searched a lot,but find no answer in the "pandas: powerful Python data analysis toolkit Release 0.19.2"</p>
  
  <p><code>df['brth_dt'].head()</code></p>
  
  <pre>
  cst_id  
  1       1976-09-23
  2       1967-01-09
  3       1990-08-04
  </pre>
  
  <p>thanks a lot</p>