CREATE TABLE captain (Captain_ID INT, Name TEXT, Ship_ID INT, age TEXT, Class TEXT, Rank TEXT)
CREATE TABLE Ship (Ship_ID INT, Name TEXT, Type TEXT, Built_Year FLOAT, Class TEXT, Flag TEXT)

Return the average age of captains in each class, and rank Y-axis from low to high order please.
SELECT Class, AVG(age) FROM captain GROUP BY Class ORDER BY AVG(age)
SELECT "Class", AVG("age") FROM "captain" GROUP BY "Class" ORDER BY AVG("age") NULLS FIRST