Calculate the average age of people who have apps installed but are not active on their devices.
average age = AVG(age); installed refers to is_installed = 1; not active refers to is_active = 0;
SELECT AVG(T1.age) FROM gender_age AS T1 INNER JOIN events_relevant AS T2 ON T1.device_id = T2.device_id INNER JOIN app_events_relevant AS T3 ON T2.event_id = T3.event_id WHERE T3.is_installed = 1 AND T3.is_active = 0