We really need a middle zone… Human brains don’t reach full maturity until around age 25 when the prefrontal cortex is done developing, and quite frankly I think it could be argued that the thing that makes a human a human is the prefrontal cortex. However, that part of the brain “turns on” at the onset of puberty. It takes about 12 years for the human brain to really master the whole controlling a human body thing, and another 12 for it to master the whole thinking and conceptualizing and thinking ahead (and a bunch of other stuff). That second 12 year span should be treated differently than both the first span and adulthood.
25 isn’t a hard line. The reality is that our brains continue to change forever. But, to use a metaphor, around our mid 20s is when it’s done “cooking”, but just like you might let meat “rest” on the counter for a bit after it’s done cooking, your brain keeps changing, just not to the same degree. Maybe some day if brain scanning technology gets better, and we have a real healthcare system, people could get scans to see when their brain seems to have reached full maturity, especially if they’ve committed a crime. Of course “full maturity” will always be sort of an arbitrary choice because as I said, our brains are never truly finished.