Decisions, Decisions: When should we upgrade from Drupal 6?
Drupal 6 was initially released in 2008. For reference, Drupal 7 was released in 2011 and the Drupal 8 estimated release is sometime in late-September (subject to change). Because of the effort required to support 3 versions of Drupal at a time, once Drupal 8 is released, Drupal 6 support will end 3 months later. The original plan to spin down Drupal 6 support on Drupal 8's release was revised to help people transition. Read more about Decisions, Decisions: When should we upgrade from Drupal 6?