SELECT DISTINCT * FROM
(
    SELECT
            c.CustomerID,
            CONCAT(c.FirstName, ' ', c.LastName) AS [FullName],
            DATEDIFF(YEAR, c.DateOfBirth, '20160101') AS [Age]
    FROM Customers AS c
    INNER JOIN Tickets AS t
    ON t.CustomerID = c.CustomerID
    INNER JOIN Flights AS f
    ON f.FlightID = t.FlightID
    WHERE f.Status = 'Arrived'
) s
WHERE s.Age < 21
ORDER BY s.Age DESC, s.CustomerID