States vary in what they consider legal/adult age, but I believe 21 is nationwide for tobacco products now.
Regarding your second and third paragraphs- yes, that's logical.. and my point exactly.. the discrepancies not in the definition of 'adult' but defined differently for different purposes.
https://eji.org/news/13-states-lack-minimum-age-for-trying-kids-as-adults/
Yes, I remember the Eric Smith case (he was 13), but I thought the M. brothers were adults at the time, I might be mistaken.
The point, though, is if kids 12-13 yrs old can be held criminally liable and prosecuted as adults because they're considered responsible for their actions, why would someone like the girl in the OP
not be considered responsible for
her actions?
I certainly don't approve of situations like the OP, but I don't think considering the girl blameless is the right approach.