Well she is under-age and I agree with Red to make sure you pass on a screen-name to the admins. I know I started reading 'mature' fiction when I was about 16, but then it wasn't a huge deal. It was a somewhat big deal but not like it is now. (That was 4 years ago for whoever is wondering.)
It's nice to see she complimented you, not many kids do that anymore. However if she is underage I would probably send her a message stating that on normal circumstances it is meant for adult viewing only and she needs to be careful reading that type of thing.