What is the average age of competitors who participated in 1988 Winter?
AVG(age) where games_name = '1988 Winter';
SELECT AVG(T2.age) FROM games AS T1 INNER JOIN games_competitor AS T2 ON T1.id = T2.games_id INNER JOIN person AS T3 ON T2.person_id = T3.id WHERE T1.games_name = '1988 Winter'