input Numbers

HTML

<script src="http://knockoutjs.com/downloads/knockout-2.2.0.js"></script>
<input type="text" data-bind="value: Age, inputFilter: 'Numbers', valueUpdate: 'afterkeyup'" maxlength="3" /> <br />
<span data-bind="text: Age"></span>

JavaScript

ko.bindingHandlers.inputFilter = {
        init: function (element, valueAccessor, allBindingsAccessor, viewModel) {
            var filter = ko.utils.unwrapObservable(valueAccessor());

            if (filter == 'Numbers') {
                $(element).keydown(function (event) {
                    // Allow: backspace, delete, tab, escape, and enter
                    if (event.keyCode == 46 || event.keyCode == 8 || event.keyCode == 9 || event.keyCode == 27 || event.keyCode == 13 ||
                    // Allow: Ctrl+A
            (event.keyCode == 65 && event.ctrlKey === true) ||
                    // Allow: home, end, left, right
            (event.keyCode >= 35 && event.keyCode <= 39)) {
                        // let it happen, don't do anything
                        return;
                    }
                    else {
                        // Ensure that it is a number and stop the keypress
                        if (event.shiftKey || (event.keyCode < 48 || event.keyCode > 57) && (event.keyCode < 96 || event.keyCode > 105)) {
                            event.preventDefault();
                        }
                    }
                });
            }

        },
        update: function (element, valueAccessor, allBindingsAccessor, viewModel) {
            // we don't need update.
        }
    };

function CarViewModel (){
   this.Age = ko.observable(10);
};

var car = new CarViewModel();
ko.applyBindings(car);