Can’t save custom post type fields

Try debug

add_action('save_post', 'add_fields_contest', 10, 2 );

function add_fields_contest( $id, $post ) {
require_once(ABSPATH . 'wp-includes/pluggable.php');
if ( $post->post_type == 'contest' ) {
    update_post_meta( $id, 'userid',                $_POST['userid'] );
    update_post_meta( $id, 'title',             $_POST['title'] );
    update_post_meta( $id, 'name',          $_POST['name'] );
    update_post_meta( $id, 'age',           $_POST['age'] );
    echo 'Saving data to post id: '.$id;
    var_dump($_POST);
    die();
}

I will recommend “Advanced Custom Fields” plugin