title: How to assign values to levels in factors?
body: <p>I want to assign values to the levels of a factor.<br>
  For example, if > 22 = mature, if &lt;= 22 = adolescence. How can I do this?   My data is:</p>
  
  <pre><code> x &lt;- factor(c(13, 18, 35), levels = c("adolescence", "mature"))
  </code></pre>