What is the average age of the survey respondents in the United States?
Average age refer to AVG(AnswerText) where questionid = 1; respondents in the United States refers to AnswerText = 'United States' where questionid = 3
SELECT CAST(SUM(T1.AnswerText) AS REAL) / COUNT(T1.UserID) FROM Answer AS T1 INNER JOIN ( SELECT T1.UserID FROM Answer AS T1 INNER JOIN Question AS T2 ON T1.QuestionID = T2.questionid WHERE T2.questionid = 3 AND T1.AnswerText = 'United States' ) AS T2 ON T1.UserID = T2.UserID INNER JOIN Question AS T3 ON T1.QuestionID = T3.questionid WHERE T3.questionid = 1
database name:db_id mental_health_survey
Question table Question Cols: question text dtype text, question id dtype integer, 
Survey table Survey Cols: Survey ID dtype integer, Description dtype text, 
Answer table Answer Cols: Answer Text dtype text, Survey ID dtype integer, User ID dtype integer, Question ID dtype integer, 
