How many users who are under 30 years old use device model of Galaxy Note 2?
under 30 refers to age < 30;
SELECT COUNT(T1.device_id) FROM gender_age AS T1 INNER JOIN phone_brand_device_model2 AS T2 ON T1.device_id = T2.device_id WHERE T2.device_model = 'Galaxy Note 2' AND T1.age < 30