For the male users no older than 18, how many times have they given the highest rating?
Male users refers to u_gender = 'M'
SELECT COUNT(T1.movieid) FROM u2base AS T1 INNER JOIN users AS T2 ON T1.userid = T2.userid WHERE T1.rating = 5 AND T2.age < 18 AND T2.u_gender = 'M'