Age of Criminal Responsibility is always a difficult slope. It would need to be considered on a case-by-case basis, but if mens rea can be proved then the juvenile should be charged as an advlt for serious crimes in my opinion. Here in Australia our system states that a child under 10 cannot be charged with a criminal offence as they supposedly cannot tell the difference between right and wrong, and we presume this unless proven otherwise for 10-14.
However if we're talking about say, a 17 year old, then it's a little different. At 17, you're well aware of what is right and what is wrong. At 17, if you go on a murdering spree, you're aware that what you're doing is wrong, you're aware that you're commiting murder (unless you have a condition that would obviously change the considerations of the case) and you should therefore in my opinion be charged as an advlt. Even though a teenager is undergoing significant development, from 14 onwards you're well aware of what is wrong, and if you willingly commit the crime anyway, then the courts have grounds for mens rea. If a 16 year old commits a serious offence, it shouldnt be wiped from their record when they reach advlthood.
Then again, Australia hasnt had a shooting spree since we brought in incrediblty successful gun laws after Port Arthur in 1996, so our experience and reactions to such matters is different to the U.S.