I've just found a nice site for learning Japanese (and other subjects). This would give you a rough idea about how much Japanese the average Japanese kid would know in each school grade.
The site:
Yahoo!
1. Select suject. Click 国語 if you want to select Japanese for instance.
2. Select topic. Click 文の組立て, for example, if you want to study Sentence Structure.
3. Select school grades (1&2, 3&4 and 5&6). If you're already fluent, you may try the junior high school level at bottom left.
Suppose you want to try grades 3&4. Click スタート to the right of 文を作る under 中級 （小３～小４）.
You wlll see a sentence with a (blank) in it. Below it, you will see the three answer choices. Click the one you think is correct. It wil give you
〇 when correct and
X when incorrect. It will give you 10 questions automatically.
________
Trying the subjects such as 理科(sciences) and 社会(social studies) would actually be a good way of testing your Japanese application abilities.
Have fun! ごちゃごちゃ考えず、とりあえず遊んでみよう！
