Write SQL query to solve given problem: What is the average age of Norwalk clients?. Keep the solution inside sql tag ```sql [SQL-Query] ```
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'