LMAO same. My mother gave me Game of Thrones to read when I was very young (idk exactly how young, but not more than 12.) My father let me watch the TV show. To be fair to my mum, she had absolutely no idea what it was about, she just knew I loved reading fantasy and that ASOIAF was a really popular fantasy series.
Personally I think Mistborn is alright though, my little brother is 11 and I would have zero problems with him reading it.
Edit: and he listened to the first two Stormlight audiobooks, so I have proof of his ability to digest adult-ish themes. Honestly I'd rather he read books than what he usually does, which is playing video games all day.
Edit 2: also I really enjoyed ASOIAF but thought it was too dark (even told my mother that) which I think is a testament to younger me's good taste in books.