What is the average age of the female users who uses a vivo device?
average age = AVG(age); female refers to gender = 'F'; vivo device refers to phone_brand = 'vivo';
SELECT AVG(T1.age) FROM gender_age AS T1 INNER JOIN phone_brand_device_model2 AS T2 ON T1.device_id = T2.device_id WHERE T2.phone_brand = 'vivo' AND T1.gender = 'F'