My wife often talks to people when she walks down the high street. Not button holing them or something weird but engaging in conversation as a natural part of other interactions like buying stuff, dodging out of the way, picking up something someone dropped and so on. Could be a friend, could be a stranger. Doesn't matter to her.
However if the kids go with her then to them, every utterance, every syllable out of her mouth is is an extreme embarrassment. So it seems from this anecdotal evidence the aversion to conversation outside the immediate friend/acquaintance group is learned, somehow, by urban kids while very young.
Maybe it's something learned at home because the kids don't seem to like conversation at home much either which is usually limited to mono-syllabic noises.