The Catcher in the Rye is told in flashback as Holden, age 17, narrates from a rest home in California. The action of the story proper takes place the previous year, when he is 16. He says on page 9:
I was sixteen then, and I'm seventeen now, and sometimes I act like I'm about thirteen.
The irony, of course, is that Holden looks much older than his chronological age. Since he has undergone a massive growth spurt and has gray hair, Holden looks like hes is in his 20s, old enough to get into bars and clubs in the city without being carded.
Emotionally, however, Holden acts like a pre-teen. Since the death of his brother Allie, Holden has turned conservative, wishing to never enter the teenage or adult worlds. He would rather stay a child, as they are the only non-phonies in the book, except for the nuns. According to Holden, children are more honest and innocent because they have not been corrupted by the materialism and sexual deviancy of the adult world.
So, Holden looks much older than he really is and he acts much younger than he really is in order both to relish in and escape from the adult world. In this way, Salinger achieves a kind of double movement as he juxtaposes the emotional and physical problems of his protagonist.