Sony has detailed which devices would get the brand new Marshmallow which is Google's Android 6.0 release. Sony was please to announced that the Xperia Z2, Z3, Z4 and Z5 series are being updated to Marshmallow, but this means that the older Xperia Z series and Xperia Z1, Z1 Compact and Z Ultra are off the list. Other device that are getting Android 6.0 are Xperia Z4, C5 Ultra, M4 Aqua and M5.
As always Sony has had a road-map of 3 months after the initial release to the Nexus line, which should point us toward a possible release early to mid January 2016.
Interested to see the release Sony made via their blog? Link