So I was super curious what Lord Lytton had written that would be considered too much for young boys. I couldn't find anything "controversial" by some light googling, but it's clear that the old man means this in a very very creepy way, and that's he's trying to insinuate that the narrator then must be "mature" and "not like other boys" for having read it. Something fun about Lord Lytton is that there's the Bulwer-Lytton Fiction Contest every year which looks for the worst opening line for a book every year. It's a lot of fun to check out.