Among all the users who use a vivo device, what is the age of the youngest user?
vivo device refers to phone_brand = 'vivo'; youngest refers to MIN(age);
SELECT 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' ORDER BY T1.age LIMIT 1