<form>
        <fieldset>
            <legend>Form:</legend>
            <br> Age:
            <input type="number" id="age" value="">
            <br>
            <br>
        </fieldset>
        <br>
        <br>
        <br>
        <input type="submit" value="Submit" onclick="Age()">
    </form>
    <script>
        function Age() {
            age = document.getElementsById("age");             
            if (age < 16) {                 alert("You are too young to play this game...");             } else {                
                 window.open("snake.html");
            }

        }
    </script>

Was ist hier das Problem?