I generally disagree with the notion of specific changes leading from "childhood" to "adulthood". Associating maturity and immaturity (or any other quality) to specific age groups, is simply inaccurate. Those kinds of changes are ongoing. They can occur individually at any time in a persons life, or not occur at all.

So then, I'm going to approach this question a bit differently.

One of the key ideas Holden seems to be struggling with is Purity, or rather Impurity. He perceives the vast majority of the world to be impure or corrupt, labeling people and things as phonies. This struggle is also internal, as on some level he recognizes that he is plagued by the same flaws he sees in everyone else (specifically when he spends several paragraphs describing the way he sometimes thinks about doing crumby things with a girl).

In some senses Holden's ideation is extremely mature, he recognizes the differences between love and lust and realizes that the former is preferable for its authenticity. The overwhelming orientation of society however, and even a part of himself, is directed the other way - and Holden will continue to struggle until he comes to terms with his own humanity in this sense.