I myself am a teenager, haven't read the book, but because of our ignorance "we are" (sorry to stereotype) shrouded behind age. To people, age means a lot, sometimes teens like myself feel we are ill-treated or perhaps robbed of what our true emotions endow. generalizations that most of us are stony-eyed and indifferent? :/ So can you see why he wants to be treated as an adult?
Yes, in a way it is. No matter what people have went through there is always one thing that comforted them in their childhood. Most people want that comfort back and in their heads the solution is be treated like a child (most children don't really have any worries because adults shelter them, some people want this back). Teenagers want to be treated like an adult at the same time because they want independence and free will. They want to do what they want without parents and limitations that minors face.