The whole "the brain isn't fully mature until age 25" bit is actually a fairly impressive bit of psuedoscience for how incredibly stupid the way it misinterprets the data it's based on is.
Okay, so: there's a part of the human brain called the "prefrontal cortex" which is, among other things, responsible for executive function and impulse control. Like most parts of the brain, it undergoes active "rewiring" over time (i.e., pruning unused neural connections and establishing new ones), and in the case of the prefrontal cortex in particular, this rewiring sharply accelerates during puberty.
Because the pace of rewiring in the prefrontal cortex is linked to specific developmental milestones, it was hypothesised that it would slow down and eventually stop in adulthood. However, the process can't directly be observed; the only way to tell how much neural rewiring is taking place in a particular part of the brain is to compare multiple brain scans of the same individual performed over a period of time.
Thus, something called a "longitudinal study" was commissioned: the same individuals would undergo regular brain scans over a period of mayn years, beginning in early childhood, so that their prefrontal development could accurately be tracked.
The longitudinal study was originally planned to follow its subjects up to age 21. However, when the predicted cessation of prefrontal rewiring was not observed by age 21, additional funding was obtained, and the study period was extended to age 25. The predicted cessation of prefrontal development wasn't observed by age 25, either, at which point the study was terminated.
When the mainstream press got hold of these results, the conclusion that prefrontal rewiring continues at least until age 25 was reported as prefrontal development finishing at age 25. Critically, this is the exact opposite of what the study actually concluded. The study was unable to identify a stopping point for prefrontal development because no such stopping point was observed for any subject during the study period. The only significance of the age 25 is that no subjects were tracked beyond this age because the study ran out of funding!
It gets me when people try to argue against the neuroscience-proves-everybody-under-25-is-a-child talking point by claiming that it's merely an average, or that prefrontal development doesn't tell the whole story. Like, no, it's not an average – it's just bullshit. There's no evidence that the cited phenomenon exists at all. If there is an age where prefrontal rewiring levels off and stops (and it's not clear that there is), we don't know what age that is; we merely know that it must be older than 25.
