With most of the stuff I write on HubPages (which is different from stuff I write elsewhere or under different circumstances), I write as if I'm having a conversation with a friend. It doesn't matter who/what the friend is because whatever I'm saying would be the same, regardless of who/what the friend is. It's just "some friend" that I kind of have in mind as I write. As with real friends in real life, I'm always the same me, but the subject of each conversation is different much of the time.
If I'm writing something that I'm aiming at a younger audience, I'm more likely to shift my tone/thinking to be more like what I'd say to a real young person I was having a conversation with in real life.
Sometimes, though, I don't imagine any audience at all. With some kinds of subjects/writing, I'll think more in terms of "getting what I have to say out to the world in general". When I do that (oddly, perhaps), I'm not imagining "everyone in the world". I'm imagining nobody and nothing at the other end of my writing. Instead, the relationship switches from "me and any reader" to "me and the words I'm writing". In fact, sometimes it isn't even "me and the words I'm writing". Instead, it's just "the words I'm writing".