What is the average weeks on top of volumes associated with the artist aged 25 or younger?
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
[{'avg(T2.Weeks_on_Top)': 1.0}]