JavaScript Form Validation

<html>
<head>
    <script language = "javascript">
        function validate(){
            var fname = document.forms["coursereg"]["fname"].value;
            if(fname == null || fname == ""){
                alert("Enter your name.")
                return false;
            }
            var age = parseInt(document.forms["coursereg"]["age"].value);
            if(isNaN(age)){
                alert("Enter a valid age.");
                return false;
            }
            if(age < 18 || age > 150){
                alert("Your age must be in between 18 to 150 to register.");
                return false;
            }
            var email = document.forms["coursereg"]["email"].value;
            if(email.indexOf("@")<1 || email.lastIndexOf(".")<email.indexOf("@")+2 || email.lastIndexOf(".")+2 >= email.length){
                alert("Enter a valid email address.");
                return false;
            }
            alert("Success!");
        }
    </script>
</head>
<body>
    <form name = "coursereg" onsubmit="return validate()" method = "post">
        <p>Name<input name = "fname" type = "text" />
        <p>Age<input name = "age" type = "text" />
        <p>Email<input name = "email" type = "text" />
        <p><input name = "submit" type = "submit">
</body>
</html>