I've learned that one of the requirements for YA literature is that it was written after 1968 (the year Outsiders was published).

But Huck is a young adult.

Maybe the intended audience wasn't YA?

I think you're right in that it may be a matter of perspective. At one time, both Huck Finn and Tom Sawyer were banned due to their content.

I believe

time can and often does alter our perspective!

There are several books that are considered YA that were never intended for that audience: crossovers are adopted by teens.

I think that is partly why: because Twain was writing for the general population: few people focused on writing for youth.