What are the average age of all the members of the tennis club
SELECT avg ( t3.age )  FROM club AS t1 JOIN member_of_club AS t2 ON t1.clubid   =   t2.clubid JOIN student AS t3 ON t2.stuid   =   t3.stuid WHERE t1.clubname   =   "Tennis Club"
average age of all members = AVG(age); members of the tennis club refers to joining tables with conditions related to the tennis club.