Inner Button Step counter using flexbox // source https://jsbin.com/gorelu

<!DOCTYPE html>
<html>
<head>
<meta name="description" content="Step counter using flexbox">
  <meta charset="utf-8">
  <meta name="viewport" content="width=device-width">
  <title>Inner Button</title>
<style id="jsbin-css">
.stepper-input {
  display: flex;
}

.stepper-input__input {
  width: 60px;
  text-align: center;
  border-left: 0;
  border-right: 0;
}

.button {
  cursor: pointer;
  padding: 5px 15px;
  color: #FFFFFF;
  background-color: #4EBBE4;
  font-size: 12px;
  border: 1px solid #16A2D7;
  border-radius: 5px;
}

.button--addRight {
  border-top-left-radius: 0;
  border-bottom-left-radius: 0;
}

.button--addLeft {
  border-top-right-radius: 0;
  border-bottom-right-radius: 0;
}
</style>
</head>
<body>
  <div class="stepper-input">
    <button class="button button--addLeft">-</button>
    <input class="stepper-input__input" type="text" value="24" placeholder="Age"/>
    <button class="button button--addRight">+</button>
  </div>

.stepper-input__input {
  width: 60px;
  text-align: center;
  border-left: 0;
  border-right: 0;
}

.button {
  cursor: pointer;
  padding: 5px 15px;
  color: #FFFFFF;
  background-color: #4EBBE4;
  font-size: 12px;
  border: 1px solid #16A2D7;
  border-radius: 5px;
}

.button--addRight {
  border-top-left-radius: 0;
  border-bottom-left-radius: 0;
}

.button--addLeft {
  border-top-right-radius: 0;
  border-bottom-right-radius: 0;
}</script>
</body>
</html>