I'm... kind of SJW-y, as a forewarning, but here's my take on it: it really depends on how it's portrayed both in canon and in the work itself, as well as the relative ages between the two characters. Anything younger than 15 for me is too young to be involved with an adult, and the adult can be no older than early/mid twenties. That said, I don't feel comfortable policing other people's experiences in fandom--what makes me uncomfortable can be normal and even accepted in another culture, and it's not my place to tell people what they can and can't write. In the context of fandom, drawing cutesy cuddles between two characters, one of whom isn't of age, is fine. Porn is where I tend to draw the line, but I just block and move on instead of sending the artist/writer hate.
I get that fiction does have an effect on how we think, but at the same time after a certain point the witch hunt for ideologically pure content will mean that we can't write about problematic things, even in a nuanced, subjective manner. As a writer, I want to be able to write about the things I'm interested in exploring, regardless of their acceptability. I read a lot of more literary works/classics and if we held them up to the same standards tumblr teens want to enforce upon fandom, we would be left with about 5 books. Problematic content can be fine--it's the execution that matters more to me. As long as a teacher/student relationship was given the space within the content to contemplate what that might mean, I'm good with it.
Ultimately: "don't care, not real" is too simplistic, as is "fiction IS reality". You can't come at this kind of thing with a one-philosophy-fits-all mentality.