<div class="container">
  <form>
    <ul class="flex-outer">
      <li>
        <label for="first-name">First Name</label>
        <input type="text" id="first-name" placeholder="Enter your first name here">
      </li>
      <li>
        <label for="last-name">Last Name</label>
        <input type="text" id="last-name" placeholder="Enter your last name here">
      </li>
      <li>
        <label for="email">Email</label>
        <input type="email" id="email" placeholder="Enter your email here">
      </li>
      <li>
        <label for="phone">Phone</label>
        <input type="tel" id="phone" placeholder="Enter your phone here">
      </li>
      <li>
        <label for="message">Message</label>
        <textarea rows="6" id="message" placeholder="Enter your message here"></textarea>
      </li>
      <li>
        <p>Age</p>
        <ul class="flex-inner">
          <li>
            <input type="checkbox" id="twenty-to-twentynine">
            <label for="twenty-to-twentynine">20-29</label>
          </li>
          <li>
            <input type="checkbox" id="thirty-to-thirtynine">
            <label for="thirty-to-thirtynine">30-39</label>
          </li>
          <li>
            <input type="checkbox" id="fourty-to-fourtynine">
            <label for="fourty-to-fourtynine">40-49</label>
          </li>
          <li>
            <input type="checkbox" id="fifty-to-fiftynine">
            <label for="fifty-to-fiftynine">50-59</label>
          </li>
          <li>
            <input type="checkbox" id="sixty-to-sixtynine">
            <label for="sixty-to-sixtynine">60-69</label>
          </li>
          <li>
            <input type="checkbox" id="other">
            <label for="other">Other</label>
          </li>
        </ul>
      </li>
      <li>
        <button type="submit">Submit</button>
      </li>
    </ul>
  </form>
</div>

body {
  background: #f2f2f2;
}