Return the average age across all gymnasts.
SELECT avg(T2.Age) FROM gymnast AS T1 JOIN people AS T2 ON T1.Gymnast_ID  =  T2.People_ID
SELECT avg(T2.Age) FROM gymnast AS T1 JOIN people AS T2 ON T1.Gymnast_ID  =  T2.People_ID
[{'avg(T2.Age)': 21.714285714285715}]