I have created the sample but for year there is a problem. You have just specified two digits for year. Now consider when user type 020728 now which year has to be taken 2002 or 1902 ?
This will check the date for the 1900 -1999
<html xmlns="http://www.w3.org/1999/xhtml">
<head runat="server">
<title></title>
<script type="text/javascript">
function DateChanged(txt) {
debugger;
var today = new Date();
var dob = txt.value;
var months = (today.getMonth() - dob.substring(2, 4) + (12 * (today.getFullYear() - (parseInt("19") + dob.substring(0, 2)))));
var age = Math.round(months / 12);
alert("Your age is " + age + "");
}
</script>
</head>
<body>
<form id="form1" runat="server">
<div>
<asp:TextBox ID="txtDate" runat="server" onchange="DateChanged(this);" />
</div>
</form>
</body>
</html>