What is the favourite movie genre for audiences of age 18?
The audience and users are the same meaning
SELECT T1.genre FROM movies2directors AS T1 INNER JOIN u2base AS T2 ON T1.movieid = T2.movieid INNER JOIN users AS T3 ON T2.userid = T3.userid WHERE T3.age = 18 GROUP BY T1.genre ORDER BY COUNT(T1.movieid) DESC LIMIT 1