That's a question! My granddaughter started in kindergarten and then started maybe 4 times. Still the same words. The problem is that it never follows anything. He goes from kindergarten to school and starts again. So it doesn't matter what age is, it depends on whether the child has a secure continuation. When another language is spoken at home, it is best that the child learns quickly and not only learns but also uses it. When there is no continuity, it is nonsense.