Teens and 20 somethings used to mix in social situations all the time. They knew each other. They dated each other. (I know, I was there, in the 80s) None of that is true anymore. Young adults have been artificially separated into two groups alien to each other. 30 year olds talk about 20 year olds as if they are a different species, and they are literally disgusted by the idea of kissing someone that young. Hostile tribes have been created.