title: Find all Users that bought their car before they were 18
body: <p>I want to find all Users that bought their car before they were 18.</p>
  
  <p>I need a query like this:</p>
  
  <pre><code>User.includes(:car)
      .where(car: { bought_at &lt; (user.birthday + 18.years)})
  </code></pre>
  
  <p><strong>How can I write such an query in Rails 5 with Postgresql?</strong> Thank you</p>