Yep, I really don't get it. I don't know about others, but since I was 15 I traveled all over the country all by myself(video game tournaments), and made decisions for myself as well, changed high schools at 17 as well. And a 17-year-old isn't a "child", and a 18-year-old isn't magically an "adult". It's individual for every single person. But frontal lobe development doesn't finish until late 20s for most, which is when I'd begin considering someone an adult. But I don't consider a 17-year-old a child. I would a 10-year-old.
A 17-year-old should be able to take responsibility for their own decisions and you can't just brush everything aside with "well she's just a child" or something ridiculous, I'd be furious if my mother thought so of me back then for example.