My daughter is nearly 6, and she still believes that the characters are real. Or at least, that's what she wants to believe. She loves the "magic" of it all, and I would hate to destroy that for her. I was maybe 6 when a friend told me that the Disney characters were just people in costumes, and I remember arguing with her that no, they were REAL. Because I WANTED them to be real. I think my daughter is much the same.
She has asked why the face characters don't look like the cartoons in the movies, and we've told her it's because those are animated movies BASED on real people. And that the princesses and face characters you meet in the parks are the actual people those movies are based on. It also helps explain why Idina Menzel sounds like Elsa. We can truthfully say that Idina Menzel is just the singer/actress who voices Elsa in the movie, but that the REAL original Elsa who the movie is based on, is the Elsa you meet in Epcot.
As far as the fur characters, that's a little trickier. I think she's already figured that out, but she wants to play along. We've always just told her that the Mickey is Mickey because Disney is magic, and just like Santa, we don't always understand how magic works - just that it does. She has asked many times though why the fur characters don't talk, and I honestly never know how to explain that one.