What makes a book for adults?

I just finished reading To Kill a Mockingbird, and it was really good book. I enjoyed it very much, but I have something to say about it. I was actually reading another question on here about how they liked the book and one person answered that they didn't like it because it didn't seem like a book for adults. That because the book was in a little girls POV that it wasn't an adult book. What do you guys think? I think that the story in Scout's POV made it all the more interesting probably because I'm still a kid myself. It was relatable to me even if it was not to other people. Do you think that if a book doesn't have complicated vocabulary or fancy wording that it isn't a book for adults? Because To Kill a Mockingbird is a pretty easy read, but it is most definetly considered a clic.