Honestly, it's starting to happen now. If not now, a year ago. I'm 14, and live in one of the most touristy cities in the world - Orlando, Florida. I was raised not only to not talk to strangers, but to be afraid of them. I live in a wonderful neighborhood that's on the safer side of Orlando, and gated. But when I was growing up, there was always the stuff on the news from the "ghetto" areas of people being raped and battered.
I don't remember the extent I understood this to when I was little, but now that I do, I feel I'm always judging people before I know them, thinking "Oh, this person could potentially be dangerous."
Here's what really went wrong though: We all grew up and realized how different we are from everyone else. Everyone got caught up in their own stereotypes, and hardly anyone crossed over from one to another. In elementary school, no one cared about what you wore, how your hair looked, or if you were good at sports. Now that we're older and it seems to be everyone's top priority, it stops people from socializing.
I'm all up for fixing it. I smile at most people I walk past. I say hi to people my age that seem cool. I have no problem with sitting anywhere at lunch. I always thought people would get over this around high school age, but apparently not.