>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.