Database: olympics
Question: What is the average age of the athletes from the United States of America who competed in the 2016 Summer Olympics?
SQL: SELECT AVG(T2.age) FROM games AS T1 INNER JOIN games_competitor AS T2 ON T1.id = T2.games_id INNER JOIN person_region AS T3 ON T2.person_id = T3.person_id INNER JOIN noc_region AS T4 ON T3.region_id = T4.id WHERE T1.games_name = '2016 Summer' AND T4.region_name = 'USA'