I like your categories. I never think about underage readers when I post a story. I frankly do not care who reads whatever there is to be read, and although there are practical reasons for age restriction on the journaling sites, it does not make sense from the writer's standpoint to limit the audience/readership by the MPAA standards (which are ridiculous to say the least), and ineffective as applied. The meat of the system you outline is excellent, when paired with appropriate content warnings.