How many competitors were there who participated in 2000 Summer with age 31?
in 2000 Summer refers to games_name = '2000 Summer'; age = 31
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 = '2000 Summer' AND T2.age = 31