As someone else said in other words, stressing other aspects: lack of inhibition has to do with tolerance for risk-taking, not for moral judgement. Plenty of adults take risks I would never take (and vice-versa).
(And I would expect lack of inhibition would correlate more with different size and development of brain structures; not myelination all by itself. Just about all decisions are balancing different stimuli, and all else being equal, even myelination wouldn’t affect that – it’s primary thing is just making those parts work better.
Now, if the myelination is uneven (which appears to be true), hitting the so-called “primitive” parts first (plausible, but haven’t seen that spelled out), that might be an argument for impulsivity as the so-called “higher” parts can’t use higher reason to quickly enough override decisions of the lower parts. But I don’t think you need to dive in to myelination at all – it’s well known that teenagers have poor impulse control.)
But impulsivity is only a good excuse for things that happen quickly. (Which I think is why several people have pointed out the length of time of the scheme. There were apparently plenty of opportunities to reconsider whether this was a good idea at leisure.
In any case, for good reason, the legal defenses around morality center around “did you know or should you have known that society considers this wrong”.
