Calculate age on date selection using Javascript

<script type="text/javascript">
        function DateSelectionChanged(e) {
            var today = new Date();
            var dob = e.get_selectedDate();
            var months = (today.getMonth() - dob.getMonth() + (12 * (today.getFullYear() - dob.getFullYear())));
            //alert("Your age: " + Math.round(months / 12));
            var age = Math.round(months / 12);
            if (age < 16) {
                alert("Error Age is less than 16");
            }
        }  
    </script>

<asp:TextBox ID="txtDate" runat="server" />
        <asp:ImageButton runat="server" ID="imgPopup" ImageUrl="~/Images/Calendar.png" />
        <ajaxToolkit:CalendarExtender ID="CalendarExtender1" runat="server" OnClientDateSelectionChanged="DateSelectionChanged"
            TargetControlID="txtDate" PopupButtonID="imgPopup">
        </ajaxToolkit:CalendarExtender>