|Larry Stylinson being super adorable. (source)
I found out about fandom around 2002, when I was still a kid. Tumblr didn't exist yet, most of Harry Potter fandom lived on Livejournal and fanfic sites such as FictionAlley, and the way I found out about all this was via a newspaper article about fanfiction. So in my personal experience, without even having to do any research on the subject, I know for a fact that the mainstream press has been reporting about fandom for at least ten years. In recent years, fan culture has come into the mainstream in a big way, with entertainment news and gossip magazines regularly reporting at Comic-Con, and 50 Shades Of Grey topping all the bestseller lists for months. A lot of reporting on fanfiction/fandom is still pretty ignorant and disrespectful because most of the journalists aren't involved in fandom themselves, but fanfic isn't the secret society it was twenty years ago. And I'd find it vanishingly unlikely for anyone currently starring in a fandom-popular TV show or in a band like 1D to not be aware of fandom. In fact, I wouldn't be surprised if winners of reality shows like American Idol and X-Factor were actually briefed about RPF as part of their press training, just so they wouldn't alienate fans by seeming shocked or appalled by the concept.