Write SQL query to solve given problem: Among all the users who use a vivo device, what is the age of the youngest user?. Keep the solution inside sql tag ```sql [SQL-Query] ```
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