title: Fill columns with most recent value
body: <p>I have a dataset like this in R:</p>
  
  <pre><code>Date       | ID  | Age |
  2019-11-22 | 1   | 5   |
  2018-12-21 | 1   | 4   |
  2018-05-09 | 1   | 4   |
  2018-05-01 | 2   | 5   |
  2017-10-10 | 2   | 4   |
  2017-07-21 | 1   | 3   |
  </code></pre>
  
  <p>How do I change the Age values of each group of ID to the most recent Age record?</p>
  
  <p>Results should look like this:</p>
  
  <pre><code>Date       | ID  | Age |
  2019-11-22 | 1   | 5   |
  2018-12-21 | 1   | 5   |
  2018-05-09 | 1   | 5   |
  2018-05-01 | 2   | 5   |
  2017-10-10 | 2   | 5   |
  2017-07-21 | 1   | 5   |
  </code></pre>
  
  <p>I tried <code>group_by(ID)%&gt;% mutate(Age = max(Date, Age))</code>
  but it seems to be giving strange huge numbers for certain cases when I try it on a v huge dataset. What could be going wrong?</p>