Solve the following sql problem:
What is the average age of the survey respondents in the United States?

Schema:
SCHEMA:
- Table: Question
  - Column: questiontext
    - Samples: [What is your age?, What is your gender?, What country do you live in?, If you live in the United States, which state or territory do you live..., Are you self-employed? (truncated)]
  - Column: questionid
    - Samples: [1, 2, 3, 4, 5]
- Table: Survey
  - Column: SurveyID
    - Samples: [2014, 2016, 2017, 2018, 2019]
  - Column: Description
    - Samples: [mental health survey for 2014, mental health survey for 2016, mental health survey for 2017, mental health survey for 2018, mental health survey for 2019]
- Table: Answer
  - Column: AnswerText
    - Samples: [37, 44, 32, 31, 31]
  - Column: SurveyID
    - Samples: [2014, 2014, 2014, 2014, 2014]
  - Column: UserID
    - Samples: [1, 2, 3, 4, 5]
  - Column: QuestionID
    - Samples: [1, 1, 1, 1, 1]

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