<!DOCTYPE html>
<html>
<body>
	<h1>Basic HTML form</h1>
	<form name="basicform" action="form.html" method="post" onsubmit="return validate()">
		<p>Age: </p><input type="text" name="age" placeholder="required" id="age" /><span id="age_err"></span><br />
		<button type="submit" id="submit">Submit</button>
	</form>
</body>
</html>
var check = false
function validate() {
	return check
}

form = document.forms["basicform"]
age = form.age
age.addEventListener("input", function (event) {
	document.getElementById("age_err").innerHTML = ""
	if (age.value == ""){
		check = false
		document.getElementById("age_err").innerHTML = "Age is required";
	}
	else {

		if (!isNaN(age.value))
		{
			if (age.value >= 1 && age.value <= 125)
			{
				check = true
			}
			else
			{
				check = false
				document.getElementById("age_err").innerHTML = "Age must be between 1 and 125";
			}
		}
		else
		{
			check = false
			document.getElementById("age_err").innerHTML = "Please enter a numeric value";
		}
	}
	
})

External CSS

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External JavaScript

This Pen doesn't use any external JavaScript resources.