How many events have happened on the device of the youngest female user?
youngest refers to MIN(age); female refers to gender = 'F';
SELECT COUNT(T1.event_id) FROM events AS T1 INNER JOIN gender_age AS T2 ON T1.device_id = T2.device_id WHERE T2.gender = 'F' GROUP BY T1.event_id, T2.age ORDER BY T2.age LIMIT 1