I'm a writer too, and I don't think it's your job to determine who is or isn't mature enough to read your writing. If you put it up on the web, having strangers read your writing, even if you don't want them to, is a natural result.
Look at it this way: You mark your stuff using the site's rating system, which is in place for that reason, and the readers choose which system is appropriate for them. A "teen" rating doesn't prohibit younger people from reading the work, I assume? There's no ID checking or age confirmation, it's just a guideline like the reading/interest level on print books? In that case, I think it's her right to read what she wants.
If her parents want to restrict her reading, that's their job, not yours. I'd stay out of this completely if I were you.