How many people who are below 30 and participated in the summer season?
people who are below 30 refer to person_id where age < 30; the summer season refers to season = 'Summer';
SELECT COUNT(T2.person_id) FROM games AS T1 INNER JOIN games_competitor AS T2 ON T1.id = T2.games_id WHERE T1.season = 'Summer' AND T2.age < 30