http://www.idownloadblog.com/2017/03/02/google-starts-rolling-out-assistant-to-android-nougat-and-marshmallow-smartphones/

Assistant is rolling out to English-speaking owners of Android Nougat (6.0) and Android Marshmallow (7.0) in the United States, to be followed by English in Australia, Canada and the United Kingdom, as well as German speakers in Germany.

Google has said that the feature will automatically come to eligible Android phones running Nougat and Marshmallow with Google Play Services via a software update.

They’ll continue to add more languages over the coming year.