Drupal has been around since 2001, but it really got traction since 2003-2004. Some of these early adapters Drupal have moved on to newer versions, but there are lots of companies which still uses Drupal5.x and Drupal6.x
At cmsMinds, we have worked on few of these upgrade projects from Drupal 5 to Drupal 7 and Drupal 6 to Drupal 7. Drupal does offer tools to help upgrade from earlier version but the biggest challenge lies when some of the contrib modules are no longer available or there is a custom module built on. As part of the overall upgrade project, we also do theme upgrade to have a new user-friendly, intuitive and responsive design.
We’ve been fortunate to work with a client who wanted to take Drupal6 and Drupal7 merge them and upgrade to Drupal8. Our team was very excited as it gave them an opportunity to learn but more importantly use creative solutions, which are not yet available in Drupal8. I guess these are some uncharted territories but client entrusted us with the challenges.