Prompting for review / reapproval of page content

I tend to look for simple solutions so my suggestion is to query pages, sort by modified day and display in Dashboard widget.

add_action( 'wp_dashboard_setup', 'add_old_pages_widget' );

function add_old_pages_widget() {

    wp_add_dashboard_widget( 'oldpages', 'Oldest Pages', 'old_pages' );
}

function old_pages() {

    $pages = get_pages( array(
    'sort_column' => 'post_modified'
    ) );

    echo '<ul>';

    foreach ( $pages  as $page) {

    $edited = human_time_diff( strtotime($page->post_modified) );
    $link = get_edit_post_link( $page->ID );
    echo "<li><a href="https://wordpress.stackexchange.com/questions/2035/{$link}">{$page->post_title}</a> ( edited {$edited} ago )</li>";
    }

    echo '</ul>';
}

Can be modified to hide if no pages old enough, etc, etc.