OK, after rereading Part 1, I've answered my own question.
Eric did not live all his life in the apartment. We are told he moved in only after his grandmother died. And a couple of pages later Toby, ever admiring Eric's ass, asks how a Jewish boy from who grew up in Westchester can have such a nice ass.
My confusion probably sprung from conflating Eric's apartment with Margaret Schlegel's house. Interestingly, while in Howards End Margaret grew up in the house she's losing and Ruth grew up at Howards End, and much is made of childhood homes, in The Inheritance Eric comes to his apt. and Walter to his house upstate as adults.