As I Lay Dying discussion.(Faulkner)?

I am a teacher who had to read Faulkner's As I Lay Dying this summer. I really enjoyed the book as well as the discussion with a goup of other teachers. One point of conflict we have hit has been over Vardaman. I know he is younger than Dewey Dell but it doesn't say exactly how much. I believe he is not much younger than Dewey Dell but is mentally challenged. Sometimes he seems to react like someone who is older but his whole thought process leaves me feeling he is challenged. Does anyone else feel this way?