input:invalid {
  outline: red solid 3px;
}

body {
  margin: 0.5rem;
}
pre {
  padding: 1rem;
  height: 2rem;
  background-color: lightgrey;
  outline: 1px solid grey;
}

<pre id="log">Validation logged here...</pre>
<input type="number" id="age" min="18" required />

const userInput = document.getElementById("age");
const logElement = document.getElementById("log");

function log(text) {
  logElement.innerText = text;
}

userInput.addEventListener("input", () => {
  userInput.reportValidity();
  if (userInput.validity.valid) {
    log("Input OK…");
  } else if (userInput.validity.valueMissing) {
    log("Required field cannot be empty.");
  } else {
    log("Bad input detected: " + userInput.validationMessage);
  }
});