Holden: "You know that song, 'If a body catch a body comin' through the rye'?..."Phoebe: "It's 'If a body meet a body coming through the rye'!... It's a poem. By Robert Burns."~J.D. Salinger, The Catcher in the Rye, Chapter 22 "Anyway, I keep picturing all these little kids playing some game in this big field of rye and all. Thousands of little kids, and nobody's around - nobody big, I mean - except me. And I'm standing on the edge of some crazy cliff. What I have to do, I have to catch everybody if they start to go over the cliff - I mean 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 do all day. I'd just be the catcher in the rye and all. I know it's crazy, but that's the only thing I'd really like to be." ~J.D. Salinger, The Catcher in the Rye, Chapter 22, spoken by the character Holden Caulfield

We could see that Holden has special feelings for the children characters.The only mature behaviour he is showing is toward kids.The special bond that he has with his brother Allie and sister Phoebe is showing that he really cares and desperately wants to protect innocence.Every time he meets someone ,he is curious to talk to them ,but at the same time he is scared that they will disappoint him.Like meeting the nuns who he admires for what they do, he is worried that they will ask him about his religion.
He also enjoys visiting museum,there are no surprises there.It is the place he can relax and not being judge by anyone.But still ,he decides not to go,he is scared of the possibility that he will realize that he is changing too.The perception of the museum would be different and he is scared of the idea that he has to grow up one day and his whole behaviour will be treated different way.
He is desperate to find someone who will be his "catcher in the rye",he wishes that someone will understand what he is going trough, by then, he will be the one protecting and keeping alive ideal world and kids in it.He almost gave up on people,his only hope are kids.
He is changing the meaning of the poem,the poem is about the body meeting the body,but desperate Holden sees that as a body catching the body.He sees himself as a saviour of the society and good values.