It’s official: Drupal 6 will be going the way of the dinosaurs on February 24th, 2016.
Have a look at the official announcement.
Sites still running Drupal 6 could continue to do so, but without Drupal community support. This means you’d be on your own when it comes to security issues, patches, and the like. If you’re running a dedicated or virtual dedicated server, you can of course continue running PHP 5 but eventually that too will become untenable, because if you have PCI scans the scanner may force you to upgrade PHP, for example.
Updating Drupal 6 to 7 (or 8! is not easy but we have tools in our arsenal to get it done.