This graph from Patricia Kuhls Video is Interesting.
We are seeing that the ability to become native/bilingual in any language is between 0~3-7, but even then in my experience if the child hasn't started before 3 it will be hard to gain that level. Especially If they live in a country where they are wanting to speak a minority language i.e English 英語 but the majority language is Japanese .
So learning from a Native speaker ネイティブスピーカー as soon as possible I of the utmost Importance.
Here is a link to the orignal post containing the video
Please watch the video