Teenage boys and girls partying together without adults is something many kind of societies frowned upon. They used to be scared of unwanted pregnancy a lot.

It did happened occasionally, but it was seen as something bad and stigma (especially toward such girls) could be quite high.

>Teenage boys and girls partying together without adults

I was assuming you were suggesting large parties regardless of age is a new historical phenomenon.

Although, fraternities and sororities have been around for hundreds of years and they're 17-21 or so. I'm sure they've had parties most of that time. It's probably how they started. Not sure if that counts with teen-only criteria. The concept of adult age has changed over time as well.

Humans party. Always have, always will. They of course were called "feasts" in historical times. Lack of partying is probably more of an anomaly (assuming the article is true). Of course, I would venture that even having a large online gaming or chat session would constitute a party. I mean its a group of people socializing, and I'm sure some of them are having beverages or botany of their choice, plus some hot pockets.

Sororities were around for hundreds years? Are they not offshots of fraternities after women were allowed on universities?

When I read old times, I tend to imagine either rural living or middle classes in cities or something else more common. Of course they had fun and socialized, but it was not the same as 80ties or something.

US Sororities have been around in the US since the mid-late 1800s, but that's not really the point. The point is teen-ish people have been having big parties a lot longer than "recent phenomena." Fraternities, think Greeks and Romans, pagan festivals, etc.