As a 16/17 year old boy, Holden was in that grey area where, although still young enough to be considered a "child" he really was having adult thoughts and experiences. Such as the loss of his brother Allie which marked him with a sense of deep grief that he could not come to terms with in any way. This fuels he belief that childhood must be the last frontier of innocence in life, and why he labels all adults phony.
His observations of adult behavior leads him to feel that he would rather remain a child, in conjunction with the fact that he longs for the past when his brother was alive. He has a remarkable ability to understand human nature, and questions the meaning of life more than a normal 16/17 year old would or should. This leads Holden to become trapped in his thoughts instead of living in the present moment.
Holden is plunged into a serious identity crisis after his brother's death, that is when his problems begin, Allie died three years ago in the novel, just when Holden started high school. He has failed out of all the schools he attended because he is caught in a cycle of grief from which he cannot escape.
Sadly his commentary on childhood is distorted by his mistaken perception that it is a time of carefree fun. Children have responsibilities, they go to school, and parents expect them to get good grades as they travel up the academic ladder right through high school and college.
Holden is not trying to behave like an adult, he is a confused, deeply troubled young man who needs psychiatric help to sort out his feelings of deep loss over his brother, to understand his purpose in life and to accept that life presents continuous challenges to each of us, death, loss, disappointment, no matter what it brings, each of us is required to cope and move past our loss, sadness, grief and embrace each day of life.
Holden saw in children the lost world of carelessness and innocence that he consistently mourned and missed as a result of "growing older", yet, as we know, Holden never really has "grown UP". Holden was very much a child, but most importantly, he was an immature one at that.
His little sister was apparently the only person he seemed to respect, and listen to- almost as if he let her lead him psychologically because he felt safer around her than around people his age or older.
His idea of childhood is almost as if it were something to protect, though he cannot protect the children "from growing up". He does value it, immensely, and he mostly does so because in the world of the "grown ups" the phoniness and need for responsibility are too much for him to handle.
He says, after all:
I keep picturing all these little kids playing some game in this big field of rye and all.... If they’re running and they don’t look where they’re going I have to come out from somewhere and catch them. That’s all I’d do all day. I’d just be the catcher in the rye and all. I know it’s crazy.