Actually in us-west-1 9.x is not even available <sigh>
You seem to be confused as to both PostgreSQL versioning and AWS offerings. I could deploy a 9.6.5 (one patch release behind the current 9.6 release) to us-west-1 right now if I wanted to. And saying "9.x" doesn't make sense - that versioning scheme began with 10.
David J.