What is the average weeks on top of volumes associated with the artist aged 25 or younger?
return volumes ;return artists #1 are  associated with ;return age of  #2 ;return #1 where #3 is  at most 25 ;return weeks on  top of  #4 ;return average of  #5
['select', 'project', 'project', 'comparative', 'project', 'aggregate']