Who is Phoebe in The Catcher in the Rye and why is she called "old" Phoebe?

Besides being a term of endearment, Holden's calling his sister old Phoebe indicates that he has known her for a long time, that he knows and understands her very well, and that he likes almost everything he knows about her character. He especially likes the fact that she is completely honest and natural, that she is not a phony and couldn't be a phony because she wouldn't know how. She serves as a contrast to the many phony adults he has enountered in his brief stay in New York. Both Holden and Phoebe are obviously exceptionally intelligent--like the members of the Glass family Salinger will write about later, notably in "Franny" and "Zooey"--although Holden doesn't think of himself as being intelligent. Because the brother and sister are both so intelligent, they understand each other easily and without having to engage in a lot of explanations. Because they have had good communication for years in spite of their age difference, Holden thinks of her as an old friend. Throughout the novel Holden is feeling terribly lonely and seems to be on the verge of a nervous breakdown. It is only at the end when he is briefly united with one person he loves and who truly loves him, that he experiences an epiphany which will lead to his being healed.