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"
Retrieve the average age of members of the club "Tennis Club".
[Schema (values) (types)]: | club_1 |  Student : stuid (text) , lname (number) , fname (text) , age (text) , sex (number) , major (text) , advisor (number) , city_code (number) | Club : clubid (text) , clubname (number) , clubdesc (text) , clublocation (text) | Member_of_club : stuid (text) , clubid (number) , position (text);