Twain's statement I think refers to inclusion, rather than exclusion.
However, even that makes a lot of assumptions. I sometimes help illiterate adults learn how to read, and they'll probably never get as far as some of the classics, unless they're very lucky and motivated. However, even a children's book can be an emotional event for some of these people. Does that mean they are of sufficiently simple emotional development that even a child's book can elicit a reaction? No. Had I told the story to them orally, they would have found it childish and uninspired.