CREATE TABLE music_festival (id DECIMAL, music_festival TEXT, date_of_ceremony TEXT, category TEXT, volume DECIMAL, result TEXT)
CREATE TABLE artist (artist_id DECIMAL, artist TEXT, age DECIMAL, famous_title TEXT, famous_release_date TEXT)
CREATE TABLE volume (volume_id DECIMAL, volume_issue TEXT, issue_date TEXT, weeks_on_top DECIMAL, song TEXT, artist_id DECIMAL)

Return the average number of weeks on top for volumes by artists that are at most 25 years old.
SELECT AVG(T2.weeks_on_top) FROM artist AS T1 JOIN volume AS T2 ON T1.artist_id = T2.artist_id WHERE T1.age <= 25
SELECT AVG("T2"."weeks_on_top") FROM "artist" AS "T1" JOIN "volume" AS "T2" ON "T1"."artist_id" = "T2"."artist_id" WHERE "T1"."age" <= 25