You can define a "onchange"-event, which triggers when the value of that hidden input changes. Here is an example of that: Does HTML Hidden control have any events? Like onchange or something?
But from reading the comments, that wasn't needed at all :-)
What is needed, is a logic which runs on page-load.
Your function only needs to add the disabled-attribute to the field, when the valid-value is true.
<input type="hidden" name="Valid" id="Valid" value="True">
<script>
// Define your function
function yourfunc() {
var validValue = document.getElementById("Valid").getAttribute('value');
if(validValue == 'True') {
document.getElementById("Address").setAttribute('disabled', true);
}
}
yourfunc(); // Call the function: is important, otherwise your code will never be run.
</script>
To remove the disabled-property again, you can check this thread: .setAttribute("disabled", false); changes editable attribute to false