The whole thing is deeply silly, because all you have to do is lie and say "Yes, I am old enough", just like when you want to read the porny fics on Teaspoon or ASCEM on Google Groups or whatever, and I can't really see that it does anything other than cover LJ's collective arse.
Having said which, I have gone back and changed the setting on a few of my fics, because I can just about buy that it's about helping people choose what they do and don't want to see, even though I was already doing that with the ratings and warnings in the headers and the cuts. But if it's going to become the de facto standard for how that's done I'll fall in line.
I very much approve of meyerlemon's idea of making up birthday dates for profiles, and wish I had thought of it before I went and put in my real one but set to "don't show anybody". Perhaps Who fandom should collectively set ours to 23rd November 1963?
Haven't written any fic yet this weekend, and I really ought to be doing work stuff, but the good people of F_W have managed to give me a That-Spoiler/Martha/Donna OT3 bunny, so we'll see where that goes.
ETA: It occurs me that I still don't really understand how the little thumbs-up things at Teaspoon work. Can anyone enlighten me?