I don't think those rules are set in stone, I think she just wants an idea of the people reading her fic and that they are of age. However, you won't know until you do ask if you haven't yet.

Even putting a note saying, "I'd like to be on your f-list in order to read your stories, but unfortunately, I'm not that active on my LJ and only really use it to comment on fanfic. Can I still be added?"

The thing is, until you ask, you'll never know and it's better to talk things with her instead of thinking you can't. It's not like she's an evil tyrant withholding fanfic, she just wants to sort of know who's reading her stuff. ;)