How can I retrieve "node author created time" ?

I have tried both

$node_author = user_load($node->uid);
$author_created_time =  $node_author->created;
$four_weeks_ago = time() - (28 * 24 * 60 * 60);
if ( $author_created_time >= $four_weeks_ago) {
    $this_user_is_new="new";
   print $this_user_is_new
 }

and

$author_created_time->user_created;
$four_weeks_ago = time() - (28 * 24 * 60 * 60);
if ( $author_created_time >= $four_weeks_ago) {
    $this_user_is_new = "new";
    print $this_user_is_new
    }

How can I retrieve the created time of the node author ?