How to force validate a valid element by jquery validation

Question

I am using jQuery validation plugin to validate form elements.By default,calling validator's form method, It won't show error message if the element is valid(of course this is right),But, when I set the element invalid,the error message won't show neither.Below is my code.

<script>
$(function () {
    $("form").validate({
        rules: {
            sex: {
                required: true
            },
            age: {
                required: true
            }
        },
        messages: {
            sex: {
                required:"Please Chose Your Sex"
            },
            age: {
                required:"Please Fill Your Age"
            }
        }
    });
    $("[name=btn]").on("click", function () {
        $("form").validate().form();
    });
});

<form>
<div>
    Sex:
    <select name="sex">
        <option value="">Chose Your Sex</option>
        <option value="1" selected="selected">Male</option>
        <option value="2">Female</option>
    </select>
</div>
<div>
    Age:
    <input name="age" />
</div>
<input name="btn" value="save" type="button" />

Step1: Click the button,sex element is valid,age element is invalid,it will be display age error message.

Step2: Fill out the age,error message will be disappear.

Step3: Select the sex to the first item(Chose Your Sex),I wish it could display the sex error message,but it will be display until the button was clicked again.

Hopefully somebody can help me!