Yes, voice is important, as well as perspective. Are you looking back on childhood? That's adult (even if the POV character is 12). Are you in the moment, with only the knowledge of a 12YO as the POV character? That's MG.
I think there is actually a sort of connection between adult and middle grade. In contrast with those two, YA often feels to me like it's a lone character against the world. But MG and adult can often have more of a community feel, where the POV character's connections with their environment/society carry more importance. Not sure how to explain this feeling, exactly, but more than once I've picked up a book and thought, is it MG? Is it adult?