Ah, youth! A time for fun and games and puppy love. Well, sort of. Over the years, Tom Sawyer has become shorthand for a mischievous, carefree boy. And he is, for the most part. That said, you can't forget that, no matter what audience Twain is writing for, he's looking back on his own childhood. Tom may be a kid, he may have a kid's feelings, and a kid's way of looking at the world, but, well, kids can be strange. And remember: Twain's all mixed up in there too, calling the shots, and giving the colorful commentary.
Though Twain intended The Adventures of Tom Sawyer to be "mainly for the entertainment of boys and girls," and only secondarily for adult readers, Tom's story is invested with so much nostalgia and so much feeling for youth that it transcends the boundaries of children's literature.
Though he remains a child at book's end, Tom is not the same person at the conclusion of the book that he is at the beginning.