How many 20 years old athletes were there in the 1984 Summer Olympic Games?
20 years old athletes refer to person_id where age = 20; 1984 Summer Olympic Games refer to games_name = '1984 Summer';
SELECT COUNT(T2.person_id) FROM games AS T1 INNER JOIN games_competitor AS T2 ON T1.id = T2.games_id WHERE T1.games_name = '1984 Summer' AND T2.age = 20