Simple AJAX issue

I'm new to AJAX + Drupal, I can't solve the following simple problem, please advise.
I have a table with people names and ages. I'd like to list the names of people on the page, by default ordered by name. Then if the link 'sort by age' is pressed, they should be listed by age without page reload.

My module:

drupal_add_js("./sites/default/modules/my_module/my_module.js");

function my_module_menu() {
  $items['my_module'] = array(
    'page callback' => 'my_module_all',
    'type' => MENU_CALLBACK,
    'access arguments' => array('access content'),
   );
  return $items;
}

function my_module_all(){

	if (isset($_POST['ord'])) $ord = $_POST['ord']; else $ord = 'name';

	global $page_content;

	$page_content .= "<div id=\"mydiv\">";
	$page_content .= "<a href='#' class='order_button' id='name'> sort by name </a>";
	$page_content .= "<a href='#' class='order_button' id='age'> sort by age </a><br>";	

	$list = get($ord);
	
	$page_content .= $list."</div>";	

// does not matter if I comment it out or not
/*
       if(isset($_POST['ord'])) {
             header("Content-type: application/json");
             drupal_to_js($list);
	}
*/	
	return $page_content;	

}

// returns the name of people ordered by name or age
function get($ord){
	// simple table with 2 columns: name, age
	$query = "SELECT name FROM people ORDER BY %s";
	$result = db_query($query, $ord);
	while ($objects = db_fetch_object($result)) {
			$list .= $objects->name."<br>";
	}
	return $list;
}

And my js:

Drupal.behaviors.my_module = function(context) {
  $('a.order_button:not(.mymodule-processed)', context).addClass('mymodule-processed')
  .bind('click', function(){
    $.post('my_module', {"ord": this.id}, callback_result);
    return false;
  });
}

var callback_result = function(response) {
  var result = Drupal.parseJson(response);
  $('#mydiv').html(result.data);  //works only with 'data' written
}