Holden Caulfield hasn't grown up since the day his brother Allie died. He's constantly preoccupied with the event; whether it's an active reminiscence or a subconcious wisp, Allie is always there. "He got leukemia and died when we were up in Maine..." (38). He was very much affected by his younger brother dying, becoming hysterical and causing damage to himself and the windows in the family garage. "They were going to have me psychoanalyzed and all, because I broke all the windows in the garage." (39) He was clearly traumatized by the incident ( the grey hairs he boasts about are a high indication- hair loses pigment when one is extremely stressed or involved in a potentially damaging experience), and appears to have been emotionally stunted by it.
Ever since, he has simply been unable to move on. He even says about himself, in his personal introduction, "I was sixteen then, and I'm seventeen now, and sometimes I act like I'm about thirteen". (9) Acts like he's thirteen. He was thirteen years old when when Allie died. Holden keeps referring to how he can often be mature, but no one ever notices that in him. All they see is how childish he is. This is most likely because that's exactly what he is: a child in a body that's growing up without him. Notice how he's constantly saying whatever happens to be on his mind? Being brutally honest is a common trait in young children, as is his constant need to be with people. Though he dislikes Ackley and Stradtlater, he still hangs out and talks with them. He eats lunch with the two nuns, taking the time to discuss literature with them. He revisits Mr. Antolini and his wife (though that ends badly). He talks to Carl, to Sally, even a grumpy taxi-driver. He's constantly moving from person to person, subconciously trying to fill the gap that Allie left.
Holden is uncertain, and scared to grow up. He hasn't been able to fully let go of Allie, but he's still reaching for higher things. He tries to go places, but has to keep up a mantra to his brother to keep him safe.
Holden still needs a hand to hold. He's lost himself in losing his brother, and he's too young mentally to find his way alone. He needs to get over the loss of Allie, and take the path of the young man that's been waiting for him since he deviated from it three years prior.