In which city was the game held where the oldest competitor participated?
in which city refers to city_name; the oldest refers to MAX(age);
SELECT T4.city_name FROM games_competitor AS T1 INNER JOIN games AS T2 ON T1.games_id = T2.id INNER JOIN games_city AS T3 ON T1.games_id = T3.games_id INNER JOIN city AS T4 ON T3.city_id = T4.id ORDER BY T1.age DESC LIMIT 1