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
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 < = value
Return the average number of weeks on top for volumes by artists that are at most 25 years old .