What is the average age of Norwalk clients?
average age = AVG(age); Norwalk refers to city = 'Norwalk';
SELECT CAST(SUM(T1.age) AS REAL) / COUNT(T1.age) AS average FROM client AS T1 INNER JOIN district AS T2 ON T1.district_id = T2.district_id WHERE T2.city = 'Norwalk'