By elementary school, no children believe in Santa anymore in my neck of woods. However, they will find it hard to understand why it rains more often in the mountains even if you explain it to them (which I think they learn in 3rd grade).
I only helped my daughter acquire skills until she was five or six. I also used educational games found on the Internet (such as those on PBS or Boowa and Kwala). So, the answer is “as soon as possible.”