function YUORMODULE_node_access($node, $op, $account) {
  $type = is_string($node) ? $node : $node->type;

  if ($op == 'view') {
    global $user; // get current user
    // check user register date
    if ($user->created < $node->created) {
      return NODE_ACCESS_DENY;
    }
  }

// Returning nothing from this function would have the same effect.
  return NODE_ACCESS_IGNORE;
}