javascript has changed a lot over the years. Maybe this site might help you
http://www.quirksmode.org/js/contents.html or look at
http://www.w3schools.com/js/js_examples.asp
For a slightly different way for doing your problem (and I spent some time on it so want some one to look at it LOL) Here is my take:
Code:
<!DOCTYPE html PUBLIC "-//W3C//DTD XHTML 1.0 Strict//EN" "http://www.w3.org/TR/xhtml1/DTD/xhtml1-strict.dtd">
<html xmlns="http://www.w3.org/1999/xhtml">
<head>
<meta content="text/html; charset=utf-8" http-equiv="Content-Type" />
<title>Please enter your age</title>
<script type="text/javascript">
function checknumber()
{
var num = document.getElementById('pnum').value;
if(isNaN(num) == true){
document.getElementById('title').style.color = "red";
document.getElementById('title').innerHTML = "You must enter a valid number between 1 to 99999";
document.getElementById('pnum').value = "";
}else if((num > 99999) || (num < 1)){
document.getElementById('title').style.color = "blue";
document.getElementById('title').innerHTML = "number not between between 1 to 99999";
}else if((num <= 99999) || (num >= 1)){
document.getElementById('title').style.color = "black";
document.getElementById('title').innerHTML = "Correct";
}
}
function setFocus()
{
document.getElementById("pnum").focus();
}
</script>
</head>
<body onload="setFocus()">
<div id="title">Please input a valid number between 1 to 99999</div>
<input type="text" id="pnum" name="pnum" onblur="checknumber();" />
</body>
</html>