Actually my team at work uses it. And we're a bunch of 20-somethings.
Usually when someone says something in jest and the other person gets mad, person A inevitably says "JK, LOL??" To which usually there will be some groans and the occasional person say "OMG".
I'm not sure where all this started from, but its all done in a mocking and sort of rediculous way. I'd like to think its a bit of a statement as to how rediculous people have gotten with language and the advent of computers.
Although, the more we use it, the more I wonder...
...And its "Jay-Kay", "El-Oh-El", and "Oh-Em-Gee" over here, for the record.
EDIT: ...Oh, and then theres been "Bee-Tea-Double You" and "Bee-Are-Bee" used once or twice too...