<!DOCTYPE html>
<html>
<body>
<p>Input your age and click the button:</p>
<input id="age" value="18" />
<button onclick="myFunction()">Try it</button>
<p id="demo"></p>
<script>
function myFunction() {
var age, voteable;
age = Number(document.getElementById("age").value);
if (isNaN(age)) {
voteable = "Error in input";
} else {
voteable = (age < 18) ? "Too young" : "Old enough";
}
document.getElementById("demo").innerHTML = voteable;
}
</script>
</body>
</html>