At the beginning of The Catcher in the Rye, Holden Caulfield worries about his childhood friend, Jane Gallagher. After discovering his roommate, Stradlater, was taking her on a date, he reflects upon their friendship. He also becomes angry at him for corrupting her innocence. When they were kids, Holden and Jane would play chess together, Jane would keep her kings in the back row, something he still remembers years later. Since they were friends since childhood, Holden associates her with youth. Stradlater has a reputation for dating a lot of girls, a trait Holden knows about. After the date, Holden confronts his roommate about taking Jane into the back of Ed Banky's car. They end up fighting, and Holden loses.
Holden Caulfield is unfairly treating Jane Gallagher like a child despite being the same age. He associates her with childhood only because they knew each other when they were kids themselves. Jane is mature enough to make her own choices, so Hold should not hold her to the expectations of a child.
[P1] Holden Caulfield is worried about his friend, Jane Gallagher. [P2] Holden associates Jane with youth and childhood. [P3] Therefore, the theme of The Catcher in the Rye is innocence and childhood.
Rejecting the premises
[Rejecting P3] Jane Gallagher is only tangentially related to Holden's youth.