-- Database schema
| Question : questiontext [ TEXT ] , questionid [ INTEGER ] primary_key | Survey : SurveyID [ INTEGER ] primary_key , Description [ TEXT ] | Answer : AnswerText [ TEXT ] , SurveyID [ INTEGER ] , UserID [ INTEGER ] , QuestionID [ INTEGER ] |
-- -- What is the average age of the survey respondents in the United States?

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;