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
[{'avg(T2.Weeks_on_Top)': 1.0}]