Old

Old is a world type exclusive to Pocket Edition. It consists of a finite world with an area of 256x256 blocks barricaded with Invisible Bedrock. It can generate with any of the current biomes, but due to the limited size, it usually doesn't generate many biomes at a time. Caves generate very rarely in these worlds.

The purpose of this world type is for low-end devices that do not perform well with infinite worlds and caves.

Before 0.9.0
Old used to be the only available world type in Pocket Edition. Like the current version, it had an area of 256x256 blocks and generate caves rarely, but it also lacked many of the current biomes, generated structures and foliage colors.

Trivia

 * If one goes into third person mode, goes to the edge of the world and faces away from it, the world can be seen from an X-Ray perspective, easily being able to see any mining systems made by the player.