test1

SELECT avg(Tmp_1.summ)
FROM (
    SELECT strftime("%m, %Y", Purchases.DATE) AS MONTH, SUM(Items.price) AS summ
    FROM Users
    INNER JOIN Purchases ON Users.userid = Purchases.userid
    INNER JOIN Items ON Purchases.itemid = Items.itemid
    WHERE (Users.age >= 18) AND (Users.age <= 25)
    GROUP BY MONTH) AS Tmp_1;

SELECT avg(Tmp_2.summ)
FROM (
    SELECT strftime("%m, %Y", Purchases.DATE) AS MONTH, SUM(Items.price) AS summ
    FROM Users
    INNER JOIN Purchases ON Users.userid = Purchases.userid
    INNER JOIN Items ON Purchases.itemid = Items.itemid
    WHERE (Users.age >= 26) AND (Users.age <= 35)
    GROUP BY MONTH) AS Tmp_2;

SELECT avg(Tmp_3.summ)
FROM (
    SELECT strftime("%m, %Y", Purchases.DATE) AS MONTH, SUM(Items.price) AS summ
    FROM Users
    INNER JOIN Purchases ON Users.userid = Purchases.userid
    INNER JOIN Items ON Purchases.itemid = Items.itemid
    GROUP BY MONTH) AS Tmp_3;