Holy shit. I rarely post in this topic, because I know little of American politics, but this case hits pretty hard for me, and is similar enough to the British system for me to comment.
I never understand how crimes as extreme as those can be mostly written off on the grounds of being too young. There is no such law over here that allows children/teenagers to be charged as adults, and I can never understand why. My sister and I actually wanted to start a campaign in favour of such a law, and compiled a list of about 250 murders committed by people aged 19 and below.
Seriously, all of those deserve to be charged as adults, without a doubt. They're clearly not learning from the previous charges against them, since their response to being arrested for theft is... to set someone on fire. :/
'Course, it's probably interesting to look at the past here, albeit slightly impossible. I can't help wondering what exactly happened to these kids to make them think that burning someone is perfectly acceptable. ¬_¬
As for "how young is too young for an adult charge," I think there is no such thing as 'too young' and that it all comes down to awareness and the extent of the crime. If someone is fully aware of what they've done, regardless of age, I think it should at least be considered as an adult crime, even if the sentence doesn't follow through with that.