a private language built to keep others out
They had their own dialect and codes, in-jokes inside in-jokes.
Context
The E.T.A. sixteens as a communication system engineered for exclusion: "their own dialect and codes, in-jokes inside in-jokes," language whose function is the sealing-off of everyone not inside it. And the note pins why — they are the age when the question turns from "Is anything true" to "Am I true, of What am I," self-opacity so acute it makes them strange, the coded tongue a wall thrown up around selves that can't yet read themselves.