function disallow_before_date_published( $can_access, $user_id, $post_id ) {

    $published = get_the_date( 'U', $post_id );
    $userdata  = get_userdata( $user_id );

    if ( strtotime( $userdata->user_registered ) < $published ) {
        $can_access = false;
    }

    return $can_access;

}

add_filter( 'wpf_user_can_access', 'disallow_before_date_published', 10, 3 );