I'm 40, but the only place I have ever abandoned proper casing is in the specific case of single-sentence text/chatroom messages (and later, 4chan posts), where I also omit the period. And even then, I only abandon sentence-casing - e.g. "I", proper nouns, etc are still capitalized. I never adopted that style in other places though, like forums. And I'm definitely not used to seeing entire chunks of prose in this more extreme version of that format (no uppercases at all, rather than just no sentence-casing).
Well I certainly didn't say all of us! Most people our age didn't live their whole life on forums/IRC/etc and even those who did, didn't necessarily pick that up. But also many of us did! I don't recall any forums back then where everyone did it; but I don't recall any where nobody did either.
