Because of the way InDesign documents are versioned, it's not possible to open them directly in earlier versions of InDesign, InCopy or InDesign Server. For example, a document created in InDesign 2015 cannot be directly opened in InDesign 2014 or CS6.
There are two techniques that can be used to share files where the document creator has a higher version of InDesign:
1) Customers with CS6 (specifically 8.1.0) or above, with access to Creative Cloud services, can use the Simple Saveback service which will allow them to open documents created by someone using a higher version of InDesign. There is some further information on this feature here:
2) The creator can save the document to the IDML format and distribute that to the user with the previous version. More information on this can be found here:
Note: content created using features specific to a particular version
of InDesign are sometimes modified or omitted if the document is opened in a
previous version. Carefully review any document opened in a previous version.