I describe Runewriters' rating as "PG-13, for a bit of language and a bit of blood."
I debated a lot on how much swearing I wanted in RW -- when I was a young teen, strong language could keep me from being able to read a thing. But ultimately I decided the occasional swear early on is a heads-up of what kind of story this is, making it clear that it isn't aimed at younger kids before the actual story hits more intense points.
I feel like beyond that it's up to readers or their parents to determine whether it's an appropriate story for them. PG-13 isn't an minimum age or intended age group, I just use it to mean "the kind of stuff you'll find here is roughly at the level of what you'd see in PG-13 stuff"
I did try to make a story that I would've been really into as a teen, but I never intentionally aimed it a specific age group -- that part isn't something I really put much thought into!