According to this, they start learning kanji and kana from Grade 1. So around 5 or 6 years old. A quick Google should answer most of your questions. Though culturally it is interesting to know how kids in a completely different culture learn to be literate. read more
The Japanese education system is a three-part system which begins at age six, and usually continues to age eighteen. Children attend elementary school, shōgakkō (小学校), for six years, starting at age six. read more