Generally accurate, with a couple of points to clarify.
First, it's not the limbic system that presents the ability to manage risk, but the executive center (in the prefrontal cortex). The limbic system is more involved in emotional control and evaluation, and is what the executive center balances. We all use the limbic system as the final arbiter of most decisions, but the adult brain is able to temper it with the long-term consequence considerations of the executive center. That's just me nit-picking, though, because your point is valid.
Second, it's not a cut-off point kind of thing, but a graduated scale. At 15, we generally have poor executive control (the reason so many teens, when asked "why did you do that" reply "I don't know"). By 25, we generally have pretty good executive control, with a fairly steady increase between those ages (ages, of course, variable by individual brain development). 18 isn't the optimal place to draw the line if we consider only maturity, but it's better than 15. 18 isn't an optimal place to draw the line if we consider only physical skill, but it's better than 25. I think for those who want a cut-off (and who consider BB to mean more than a starting point), 18 is a reasonable place.
Well, it's the relationship between prefrontal cortex and limbic system. You're correct with what your saying, but the limbic system is not fully developed either, which prevents them from making rational decisions even if they are aware of what the rational decision.
Regarding the 25 thing, if those parts of the brain being developed enough was a requirement for black belt for some reason, most people would probably have it developed enough by the time they are 19-21 where they would pass the requirement. Some people have delays and wouldn't be ready until 23/24, while others on the other end would be ready by 17/18. The issue is, there is no real way to tell where a person is at in that development, if you're only seeing them in the dojo where I assume they are not taking many risky decisions (if they are, that's another problem), so in order for you to know the person fulfills that requirement, outside of examining them in their every day life, you would have to raise it to that 24 year old who is delayed for whatever reason.
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