Of the first 60,000 customers who sent a true response to the incentive mailing sent by the marketing department, how many of them are teenagers?
RESPONSE = 'true'; teenagers are people aged between 13 and 19 years;
SELECT COUNT(T1.ID) FROM Customers AS T1 INNER JOIN Mailings1_2 AS T2 ON T1.ID = T2.REFID WHERE T1.age >= 13 AND T1.age <= 19 AND T2.RESPONSE = 'true'