Write SQL query to solve given problem: What is the age of the oldest active user that participated in the event held on 5/6/2016 at coordinates 121, 31?. Keep the solution inside sql tag ```sql [SQL-Query] ```
SELECT T3.age FROM app_events AS T1 INNER JOIN events_relevant AS T2 ON T1.event_id = T2.event_id INNER JOIN gender_age AS T3 ON T2.device_id = T3.device_id WHERE T1.is_active = 1 AND T2.longitude = 121 AND T2.latitude = 31 AND SUBSTR(T2.timestamp, 1, 10) = '2016-05-06' ORDER BY T3.age DESC LIMIT 1