The Computer Entertainment Rating Organization (Japan's equivalent of the ESRB) reserves its most restrictive rating, Z, for games intended for ages 18 and up. There's a slightly less restrictive D rating for ages 17 and up, which gives Z-rated games a sort of stigma almost equivalent to the ESRB's AO rating -- Z-rated games require the buyer to provide proof of age, and often have little or no display presence in stores.
Which isn't exactly good news for the first CERO Z-rated DS game ... Grand Theft Auto: Chinatown Wars, whose North American sales suggest it doesn't need any more disadvantages. Like every other GTA, Chinatown Wars will carry the adults-only rating in Japan. Unlike every other GTA (except for IV on the PC), this one will be published by Cyberfront instead of Capcom when it comes out in the fall.