Knockout Validation: how to validate the fields on button click, not on input change

I'm using the following knockout validation plugin: https://github.com/Knockout-Contrib/Knockout-Validation

I want to validate my fields when I click on the "Submit" button, not everytime when I change the input's value. How can I do that?

Javascript:

ko.validation.init({
    insertMessages:false,
    messagesOnModified:false,
    decorateElement: true,
    errorElementClass: 'wrong-field'
}, true);

var viewModel = {
    firstName: ko.observable().extend({minLength: 2, maxLength: 10}),
    lastName: ko.observable().extend({required: true}),
    age: ko.observable().extend({min: 1, max: 100}),
    submit: function() {
        if (viewModel.errors().length === 0) {
            alert('Thank you.');
        }
        else {
            alert('Please check your submission.');
            viewModel.errors.showAllMessages();
        }
    },
};

viewModel.errors = ko.validation.group(viewModel);

ko.applyBindings(viewModel);

HTML:

<fieldset>    
    <div class="row" data-bind="validationElement: firstName">
        <label>First name:</label>
        <input type="text" data-bind="textInput: firstName"/>
    </div>

    <div class="row" data-bind="validationElement: lastName">
        <label>Last name:</label>
        <input data-bind="value: lastName"/>
    </div>

    <div class="row">
        <div class="row">
            <label>Age:</label>
            <input data-bind="value: age" required="required"/>
        </div>
    </div>
</fieldset>

<fieldset>
    <button type="button" data-bind='click: submit'>Submit</button>
    &nbsp;
</fieldset>

This is my jsfiddle: http://jsfiddle.net/xristo91/KHFn8/6464/