WITH
avgs AS
    (SELECT userid, AVG(price) AS av FROM Purchases
        INNER JOIN Items ON Items.itemid = Purchases.itemid
    GROUP BY userid)
SELECT AVG(av) FROM avgs
    INNER JOIN Users ON Users.userid = avgs.userid
    WHERE age BETWEEN 18 AND 25
UNION
SELECT AVG(av) FROM avgs
    INNER JOIN Users ON Users.userid = avgs.userid
    WHERE age BETWEEN 16 AND 35
UNION
SELECT AVG(av) FROM avgs;