Since the worksheets include English instructions and translations (where required), any student at any age who can read English can do the course without outside help. If the student is very young, his or her parents may have to give some help.
If you are considering entering your child into Kumon, such things as whether your child is attending a Japanese school or an international school, etc., will determine whether it is more effective to enroll in the Japanese program or the Kokugo program. Please visit a Kumon center for advice.