A Bug's Life

You can safely say that the fourth wall existed by the 1930s, when Brecht started writing stuff that breaks it ;) It seems likely that Shakespeare played with the properties of the fourth wall with his soliloquies (bearing in mind the fairly small, intimate, crowded playing spaces he wrote for). But stuff like Wilde presupposes a fourth wall, as in fact did Victorian theatre design with the proscenium arch. The curtain and the narrow run in front of it are definitely giving the impression of a wall here.

As lnr says above, I don't think there's a 4th wall there for stand-up.

As for children, I don't know. When I was little (say, 7 or younger) I saw pantomime, where there's lots of crossing of the 4th wall. I saw very few other types of play before I was in my teens, when I definitely had the concept of a 4th wall. Are we likely to get a different answer for those who grew up with TV, as opposed to those who grew up with radio/before the days of radio? I sort of assumed that the 4th wall was implicitly there, like with TV programs, and you just 'looked in' on the moment as it were. It was a revelation to me that actually, quite a lot of theatrical art is based on using that to your advantage, or showing it up as a convention etc. etc. As for who puts it there - it's both audience AND performers, coupled with the playing space which can either make it easy or difficult.