I don't think it's about organic, physical brain development, nor are there any particular implications to be highlighted, with respect to behavior or ability. That's just pop science fluff. The brain doesn't stop growing until you're 25, man! So give these kids a break. Young people typically just don't know shit, and are very poorly prepared for adult life, with pretty much regimented formal education being the extent of their post-puberty experience and training, as well as the largely socially unsupervised HS years, and so naturally they might get a little effed up, and be prone to shitty behavior. That's a given. After all, Albert Einstein released the special theory of relativity and had his 'magical year' or whatnot where he produced four seminal physics papers working out of his home office when he was 25. Bob Dylan produced the great 'Highway 61 revisited' record, and pretty much had recorded and composed all his landmark songs and lyrics all by 25 or younger that eventually won him the Nobel. Alexander the Great, well you know what he did. When he was like, 16. Mozart. The list goes on, and it really does.
So yeah, sure, the brain is not quite finished physically growing, but the effects on one's behavior and mental state is probably trivial or irrelevant. 'Nurture' so to speak, being far, far more important than nature, and it's what you actually do and learn about, the wisdom you might glean and the habits your form and the like, and the people you emulate and the particular milieus that surround and influence you, and not the physical apparatus of your brain that matters, as a young adult. Modern society has just become alarmingly effete and irresponsible in all kinds of myriad ways. And is always looking for excuses for their negligence.
They do say you shouldn't smoke a ton of weed until your brain is totally done growing, and that too much might stunt the physical development of your brain or something. But once you're safely past the 25 year old cutoff smoke all the weed you want, and know you're safe! Obviously that's just a good rule of thumb, and not really understood at all, and could be totally wrong. Anything involving the brain is basically not understood at all, at least not on a terribly significant technical level.