Yes I agree, which is probably why we should treat 18-year olds more as children than adults (although obviously they are in-between the two). Brains continue to develop to the age of about 25.
The study that appeared to show them stopping development at 25 did not have any participants older than 25.
The difference between an 18-year-old and a 24-year-old is much more comparable to the difference between a 24-year-old and a 30-year-old.
We should be treating teenagers much more as adults-in-training, in the sense of meaningfully giving them the tools to succeed as adults, rather than treating them like pure innocent children who must be sheltered from absolutely everything hard, scary, or taboo.
However, as it stands we generally do not do that—hence, in this case, she was indeed a child, and should have been given compassion, better tools, and better chances, not locked up.
After 50 years, they seem to decline.
So if 17 can't be called a child, what can? You have to draw the completely arbitrary like somewhere. Do you chose the legal 18 (in the US)? The Hebrew 13? Some other metric?
(And probably also drive, it always stuck us in Europe pretty weird you allow minors to drive in the USA as the car can be a very lethal tool).