Currently, a majority of the students happens to be under grade school kids. I have no problem whatsoever teaching those kindergartens.
When I teach them, I focus on many factors, especially the pronunciation. According to my neurosurgeons, the tendency to speak other languages (this case, English) fluently, is limited. Furthermore, based on his theories, the most appropriate age is around 8 years old.
This is just one man's opinion. I hope you don't take it so seriously. However, to certain degree, I agree with the doctor's point of view. Sooner the better.