You are using a hyperbolic example. I think it's perfectly fine to argue that the juvenile justice system should be reformed to remove that max, so that, while crimes are handled and punished differently, in edge cases like you are talking about there isn't a max punishment, but how the case is handled, and what the punishment for each individual crime is should be different. I do not think a life sentence for felony murder by a 13 and 15 year old will ever be appropriate.
Even if you believe that there are people who are pure evil, how exactly do you identify who can be reformed, and who can't? I would not be completely comfortable making that decision about most adults. The idea that we can decide a 13 and 15 year old can't reform is beyond insane to me.
We as a society have decided that children under 18 do not have the same rights and responsibilities as adults. We do that because, as a society, we understand that it takes until at least 18 for kids to learn and develop to the point that they can be full members of society.
If we decided that 13 year olds were responsible enough to vote, live on their own, drive, drink, and otherwise act as fully independent members of society, then I would be ok with them being criminally charged as fully independent members of society.