Write SQL query to solve given problem: What is the average age of first 60,000 customers who sent a true response to the incentive mailing sent by the marketing department?. Keep the solution inside sql tag ```sql [SQL-Query] ```
SELECT AVG(T1.age) FROM Customers AS T1 INNER JOIN Mailings1_2 AS T2 ON T1.ID = T2.REFID WHERE T2.RESPONSE = 'true'