C# - Enable Textbox With Checkbox Checked With Javascript?

I am trying to enable/disable textbox when checkbox is checked (enable) or unchecked (disable). WIth the piece of code i have nothing is happening once the checkbox is checked/unchecked.

Here is what I have:

<%@ Control Language="C#" AutoEventWireup="true" CodeBehind="AssociationInfo.ascx.cs" Inherits="Administration.Modules.AssociationInfo" %>
<%@ Register Assembly="AjaxControlToolkit" Namespace="AjaxControlToolkit" TagPrefix="asp" %>
<script type="text/javascript" language="javascript">
function enableTextBox() {
window.onload = function () {
var check = document.getElementById("chkAssociation");
check.onchange = function () {
if (this.checked == true)
document.getElementById("txtAddress").disabled = false;
else
document.getElementById("txtAddress").disabled = true;
};
};
}
</script>
<div>
<h2>Association Info</h2>
<br />
<asp:CheckBox Checked="false" ID="chkAssociation" runat="server" />
<asp:TextBox ID="txtAddress" Text="Test" runat="server" />
</div>


The code is in web user control. Could that be reason why is not working correctly?