While America allows anyone to buy games through the PlayStation Network, Japan is a little more strict.
In Japan, a game rated CERO Z means it is an adult game. These games aren't sold to anyone under 18, and usually consumers have to ask the store manager for them. As well, most CERO Z games were not available online, until now.
According to Siliconera, Sony Computer Entertainment Japan is now releasing certain CERO Z games on the PlayStation Network, creating an 18+ section in the process for the network.
The following games will be available in this 18+ section:
The Last of Us
God of War: Ascension
Killer is Dead
Ninja Gaiden 3: Razor’s Edge
More titles like Assassin's Creed IV will be added in the future. It is quite interesting how different the game regulations are in Japan than America.