Below is the list of tables from the database along with their columns and datatype. Need to generate an accurate SQL query for the question.
Only use the columns present in the respective table. Only use the columns which are required to generate the SQL query. Only use table joins if required and not otherwise.

### Tables ###
TABLE: CREATE TABLE Student (StuID VARCHAR, age INTEGER)
TABLE: CREATE TABLE Participates_in (StuID VARCHAR, age INTEGER)

### Question ###
Show the ids for all the students who participate in an activity and are under 20.

### Accurate SQL ###

SELECT StuID FROM Participates_in INTERSECT SELECT StuID FROM Student WHERE age < 20