Question: What is the average age of the survey respondents in the United States?
SQL: 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