Eleven meaning?

IN the plume editon, page 348, or subchapter 27 in Bridge and City, it says : Eddie felt himself shrinking toward all the fantastic, hallucinatory terrors of eleven. i recall the book saying eleven before earlier also, but couldn;t find it, either way...what does that statement mean?

It's his age -- that's how old he was when he and Henry went to The Mansion and couldn't get up the nerve to go in.