My son is almost 11 years old and enjoys singing. But, I know that the voice needs to mature and must be treated properly to reach its full potential. When is a good time for him to start private lessons?
Now would actually be a good time. Although his voice will probably start maturing, he would already have the proper training in order to maintain the notes and whatnot. He could keep practicing and pushing his voice as it matures to still be able to hit the same notes, and grow with his voice.