Younger kids will like it too!
I've tried reading chapter books with my six year old daughter before - things like the new Amelia Bedelia books and some mermaid adventure stories. She'd be into it for a chapter or two and then done. But for some reason I picked up our copy of Treasure Island and she was riveted throughout. We read three chapters a night every night until it was done, which took about a week. I'm sure that most of it went over her head, which is why we will be rereading this when she is 8 or so. But she still found the story really interesting, and was very worried about whether the pirates would win and whether Long John Silver was actually a good guy or bad guy.
We learned lots of new words, and Jim Hawkins demonstrated the importance of sticking to your word even if other people don't.