Among the users who use OPPO, calculate the percentage of those who are under 50 years old.
OPPO refers to phone_brand = 'OPPO'; percentage = MULTIPLY(DIVIDE(SUM(age < 50), COUNT(device_id)), 1.0); under 50 years old refers to age < 50;
SELECT SUM(IIF(T1.age < 50, 1, 0)) / COUNT(T1.device_id) AS per FROM gender_age AS T1 INNER JOIN phone_brand_device_model2 AS T2 ON T1.device_id = T2.device_id WHERE T2.phone_brand = 'OPPO'