Ugh, I remember that essay too! But that was definitely more or less the pattern I took, albeit with a certain degree of zig-zagging back and forth -- I'd been reading some YA/adult stuff from a very early age along with the kidlit, so it was less of a sudden jump and more of a gradual shift in proportions.

And in retrospect there was a lot of that covert YA shelved simply as genre stuff in the library I haunted as a child, too -- they had most of the mass-market paperbacks all shelved in a separate room, sorted out by genre, and I remember there were things like the early Heinlein juveniles mixed in with the more grown-up SF titles.