I just overheard my coworker say this. Language Log has a great article about this, but I think they missed the seed of the current use of this phrase by adults. I think that the contemporary use was started by Peter Bagge in his comic Neat Stuff. In it, there's a series called The Bradleys, and Babs had a story titled "You're Not the Boss of Me". Why this comic book? It was popular with the indie comics scene, which was popular with the grunge music demographic. Then, for whatever reasons, Bagge put out Hate comics, which had a whole grunge scene theme. Matt Groening (the Simpsons) also praised Bagge. A Bradley's compilation, which was published around 1990, has been reprinted, and it features that Babs story.