It's a term of the pornographer's art. I had a friend from college who wrote for Larry Flynt's Barely Legal magazine for a while. She went on to bigger but arguably less respectable things (political blogging).
Yes, "Barely legal" is used by many legal pornographers.
The article suggests some connection between the phrase "barely legal" and images of child sexual abuse.
It's clear that "barely legal" has a well-established meaning of "absolutely legal; documented adults; no chance of being under age". The article failed to establish a connection between the term "barely legal" and images of child sexual abuse.
