What is the date when the youngest user made his or her first post?
the youngest user refers to MIN(Age); first post refers to MIN(CreaionDate);
SELECT T2.CreaionDate FROM users AS T1 INNER JOIN posts AS T2 ON T1.Id = T2.OwnerUserId WHERE T1.Age IS NOT NULL ORDER BY T1.Age, T2.CreaionDate LIMIT 1