In my opinion one reason why they cannot be made fully responsible for their actions is, that they do not have the same rights as adults. They are not allowed to drive cars, to drink alcohol, to have sex, to vote, to marry, to become president and so on. Another reason might be, that they did not have the same chance to develop an ethic framework as adults had, because they had less years to learn.
> age is often used as a (bad) proxy for maturity.
I agree, but on the other hand there is a need for a fair (same for all) measure. I have no idea what would be a better objective measure.
> That said, prison time for anyone, adults or children, for computer-related crimes is barbaric.
I would agree if this is limited to "crimes" that are only or at least mostly computer related.