Glad to see this brought up, as I've been thinking about the subject, because of my son.
He was always the least bookish of our little family; when he was a little boy there would be times when he didn't have a book on the go, which was unheard of for his parents and sister. Still, by any kind of sane standards he was still a reading sort of boy - read for an hour or so before bed, at least. Then he went off to university, in a traditionally non literary discipline, and stopped reading for pleasure altogether. Partly due to lack of time, but also, I think, due to an attitude among his peers that reading books for fun was something that only those effete losers in Arts programs would do.
He's in his last year, now, and has rearranged his schedule to graduate a semester later than originally planned. The time pressure has eased up, and I think he's grown up some. All of a sudden, he's reading like he used to, and discussing books with me, like he used to.
I'm starting to think it's a lot like religion - get them early enough, and it will have a hold on them for the rest of their lives!