<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 {
  font: normal 18px/1.5 "Fira Sans", "Helvetica Neue", sans-serif;
  background: #3AAFAB;
  color: #fff;
  padding: 50px 0;
}

.container {
  width: 80%;
  max-width: 1200px;
  margin: 0 auto;
}

.container * {
  box-sizing: border-box;
}

.flex-outer,
.flex-inner {
  list-style-type: none;
  padding: 0;
}

.flex-outer {
  max-width: 800px;
  margin: 0 auto;
}

.flex-outer li,
.flex-inner {
  display: flex;
  flex-wrap: wrap;
  align-items: center;
}

.flex-inner {
  padding: 0 8px;
  justify-content: space-between;  
}

.flex-outer > li:not(:last-child) {
  margin-bottom: 20px;
}

.flex-outer li label,
.flex-outer li p {
  padding: 8px;
  font-weight: 300;
  letter-spacing: .09em;
  text-transform: uppercase;
}

.flex-outer > li > label,
.flex-outer li p {
  flex: 1 0 120px;
  max-width: 220px;
}

.flex-outer > li > label + *,
.flex-inner {
  flex: 1 0 220px;
}

.flex-outer li p {
  margin: 0;
}

.flex-outer li input:not([type='checkbox']),
.flex-outer li textarea {
  padding: 15px;
  border: none;
}

.flex-outer li button {
  margin-left: auto;
  padding: 8px 16px;
  border: none;
  background: #333;
  color: #f2f2f2;
  text-transform: uppercase;
  letter-spacing: .09em;
  border-radius: 2px;
}

.flex-inner li {
  width: 100px;
}