On February 13, 2008, Drupal 6 was released. The policy of the community is to support only the current and previous stable versions. (When Drupal 6 was released, Drupal 4.7.x was marked unsupported. When Drupal 7 came out, Drupal 5.x was marked unsupported.) This policy was created to prevent core and module maintainers from having to maintain more than 2 active major versions of Drupal.
With the coming Drupal 8 release, this policy has been questioned. We want to ensure that sites that wish to move from Drupal 6 to Drupal 8 have a supported window within which to do so.
Drupal 6 core and modules will transition to unsupported status three months after Drupal 8 is released. “Unsupported status” means the community will not be providing support or patches in the same way we do now. Continuing to support Drupal 6 would be difficult for many reasons, including a lack of automated test coverage, the requirement for rigorous manual release testing, the slow-down it introduces in the release of security fixes for the vast majority of Drupal users (on version 7+), and the general shift of volunteers in the community moving their attention onto Drupal 8 development.
Drupal 8 is expected to be released some time in the beginning of 2015.
The Drupal Project: https://www.drupal.org/node/2288521