One philosophical point:
I find it curious that we absolve most juvenile offenders of their crimes, or at least punish them to a lesser degree, due a certain recognition of their lack of education and self control, yet we rarely extend such consideration for the circumstances under which offenders over the age of 18 are raised in.
Most offenders I know were abused as children, or raised in an environment of waste, spite, poverty, ignorance and anger, or have drug and alcohol problems stemming from childhood years. It seems to me that the only difference between them and a 13-year-old offender is that they now have a few more years of a bad upbringing, and more intelligence to devote to self-loathing and contempt for society. Sure, they could apply that intelligence to changing their evil ways, but couldn't the same be said for a juvenile offender?
The age of majority just seems an arbitrary distinction. If anything, people usually get worse with age.