Trying to pin down one 'appropriate audience' for the notebook feels like missing the point entirely. It’s a tool, not a piece of media with a set demographic. My 11-year-old niece uses one for sketching cartoon characters and keeping a weirdly detailed log of her cat’s naps. Meanwhile, my retired neighbor has a whole shelf of them filled with gardening notes, bird sightings, and recipes.
I think the rating is more about the user's intent and maturity than a strict age gate. A blank notebook can hold a pre-teen's diary, a college student's lecture notes, or an adult's business plans—all of which require different levels of personal responsibility and privacy awareness. The rating likely just exists to flag that the content inside, created by the user, isn't moderated, so supervision for younger kids makes sense.
The real question is who's ready for that kind of open-ended, self-directed creativity or documentation. That readiness varies wildly person to person.