Below is an context that describes a sql query, paired with an question that provides further information. Write an answer that appropriately completes the request.  # noqa: E501
### Context:
CREATE TABLE visitor (age INTEGER, Level_of_membership VARCHAR)
### Question:
What is the average age of the visitors whose membership level is not higher than 4?
### Answer:
SELECT AVG(age) FROM visitor WHERE Level_of_membership <= 4