Once upon a time, children who committed serious crimes were treated like adults and sent to prisons and hanged and flogged, like adults.
Then we became civilized. We realized that it was horribly unjust to treat, say, a 12-year-old, the same way we treat a 25-year-old, and to hold them to the same standards of behavior and responsibility.
We realized, for one thing, that children are not complete human beings yet. Their brains are not fully developed. They are undergoing big changes. They are not “responsible” in the same way a mature adult is responsible. So they need to be treated differently. They still have a chance to become a normal, well-functioning, rational adult human being.
And then we completely forgot about all those things because we are ourselves evil beings and we want to see people suffer, given the opportunity, so now we routinely charge adolescents “as adults” so we can maximize the severity of their punishments.
There are days, by golly, when our society looks like we have already embraced Sharia law. Gather the stones!