Which city in West North Central has the highest number of customers over the age of 60?
in North Central refers to Region = 'North Central'; highest number of customers refers to max(count(client_id)); over the age of 60 refers to age > 60
SELECT T2.city FROM client AS T1 INNER JOIN district AS T2 ON T1.district_id = T2.district_id WHERE T2.division = 'West North Central' AND T1.age > 60 GROUP BY T2.city ORDER BY COUNT(T2.city) DESC LIMIT 1