Table schema:
CREATE TABLE club (clubid VARCHAR, clubname VARCHAR); CREATE TABLE student (age INTEGER, stuid VARCHAR); CREATE TABLE member_of_club (clubid VARCHAR, stuid VARCHAR)

Question: Retrieve the average age of members of the club "Tennis Club".
SQL: 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"