Drupal 6 extended support announcement | Drupal.org

drupal_logoOn 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.

Source: Drupal 6 extended support announcement | Drupal.org