According to @Ricciolo1, a well-known leakster, Sony plans to update its Xperia Z5 lineup to Android 6.90 Marshmallow in January with older models to receive the same treatment shortly after.
Marshmallow offers a bunch of improvements, optimizations and features that’d be welcome on any smartphone. Battery life improvements and performance boosts are the most noticeable changes to the software.
Sony has been pretty good about keeping older devices in the update loop. It updated theXperia Z (not the Z1) all the way to Lollipop 5.1.1, despite having an older Snapdragon S4 Pro processor.
Whether or not Sony will decide to update the Xperia Z this time around is unknown but it sure would be awesome to see Marshmallow running on a phone from early 2013.