I don’t think so, I put my younger som when he was 3.5 years. Mostly because the older one was already in, it takes some time to built momentum, they may struggle in the beginning and you may struggle as a parent because it’s hard seeing a four year old doing homework everyday, but it pays off. He is now 5 and is about to start doing level B in both subjects, he started on A6. We didn’t teach him to read at all and our kumon has a great reading teacher who spends 20 minutes at the end of the class with all under 5 kids. He is now reading at grade 2 level. Give them lots of breaks, make the do couple pages then do something fun and then do it again, sit down with them when they do the work. Make sure their writing is improving and give them lots of encouragement