Japanese gamers must be over the age of 18, if they want to play God of War III. Amazon Japan has just unveiled the official box art for the game in that country, and it's pretty much the same as you'll get elsewhere, with a big, fat Z rating on it.
It is now legally restricted to adults in Japan, while gamers in the US can play it under the slightly more relaxed M (Mature) rating, for gamers aged 17 and over. In Australia, typically seen as being very strict in terms of censorship, the game has already been considered suitable for 15 year old gamers, or younger children if they are accompanied by a parent or adult guardian.
The previous Japanese-published entries in the series (God of War, God of War II, God of War: Chains of Olympus and God of War: Collection) have been given a CERO D rating, for ages 17 and up.
One of the most eagerly-anticipated releases of 2010, God of War III is due to launch next month, exclusively for PS3.